v3.25.4
Insurance Liabilities - Undiscounted, Incurred and Paid Losses and Allocated Loss Adjustment Expenses (Details)
$ in Millions
Dec. 31, 2025
USD ($)
claim
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
U.S. Workers' Compensation (before discount)                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 6,075                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 3,004                  
Year 1 (as a percent) 11.00%                  
Year 2 (as a percent) 17.00%                  
Year 3 (as a percent) 10.50%                  
Year 4 (as a percent) 7.70%                  
Year 5 (as a percent) 4.80%                  
Year 6 (as a percent) 3.60%                  
Year 7 (as a percent) 2.60%                  
Year 8 (as a percent) 1.10%                  
Year 9 (as a percent) 1.10%                  
Year 10 (as a percent) 0.80%                  
U.S. Workers' Compensation (before discount) | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 957 $ 986 $ 1,025 $ 1,036 $ 1,075 $ 1,090 $ 1,140 $ 1,318 $ 1,346 $ 1,299
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 137                  
Cumulative Number of Reported Claims | claim 32,431                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 715 708 694 686 662 630 584 521 378 147
U.S. Workers' Compensation (before discount) | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 664 673 705 712 731 763 776 850 789  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 158                  
Cumulative Number of Reported Claims | claim 28,371                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 407 402 395 389 367 333 294 224 93  
U.S. Workers' Compensation (before discount) | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 769 786 875 896 911 961 1,021 998    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 167                  
Cumulative Number of Reported Claims | claim 22,736                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 459 448 409 388 359 296 215 85    
U.S. Workers' Compensation (before discount) | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 684 730 788 801 812 873 887      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 141                  
Cumulative Number of Reported Claims | claim 17,596                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 431 417 389 347 301 219 93      
U.S. Workers' Compensation (before discount) | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 421 434 477 521 573 597        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 86                  
Cumulative Number of Reported Claims | claim 14,305                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 279 259 245 205 159 64        
U.S. Workers' Compensation (before discount) | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 533 514 545 570 597          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 197                  
Cumulative Number of Reported Claims | claim 11,490                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 241 211 171 128 60          
U.S. Workers' Compensation (before discount) | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 496 464 493 523            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 202                  
Cumulative Number of Reported Claims | claim 10,079                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 189 143 102 45            
U.S. Workers' Compensation (before discount) | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 476 465 500              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 222                  
Cumulative Number of Reported Claims | claim 9,332                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 144 103 38              
U.S. Workers' Compensation (before discount) | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 540 567                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 280                  
Cumulative Number of Reported Claims | claim 7,683                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 104 35                
U.S. Workers' Compensation (before discount) | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 535                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 484                  
Cumulative Number of Reported Claims | claim 5,476                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 35                  
U.S. Workers' Compensation (before discount) | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 374                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 10                  
U.S. Workers' Compensation (before discount) | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 3,071                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (79)                  
U.S. Workers' Compensation (before discount) | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 3,445                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (69)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (44)                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (113)                  
U.S. Excess Casualty                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 6,215                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 3,517                  
Year 1 (as a percent) 1.10%                  
Year 2 (as a percent) 10.60%                  
Year 3 (as a percent) 8.30%                  
Year 4 (as a percent) 19.70%                  
Year 5 (as a percent) 10.90%                  
Year 6 (as a percent) 6.90%                  
Year 7 (as a percent) 6.40%                  
Year 8 (as a percent) 7.60%                  
Year 9 (as a percent) 3.50%                  
Year 10 (as a percent) 10.40%                  
U.S. Excess Casualty | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 1,305 1,317 1,276 1,263 1,250 1,274 1,171 1,162 1,146 898
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 226                  
Cumulative Number of Reported Claims | claim 3,064                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 986 851 798 670 566 502 388 204 80 28
U.S. Excess Casualty | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,270 1,228 1,182 1,200 1,157 1,153 1,097 1,002 856  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 257                  
Cumulative Number of Reported Claims | claim 2,358                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 898 860 781 676 585 505 156 45 1  
U.S. Excess Casualty | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 762 779 779 769 769 721 646 648    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 110                  
Cumulative Number of Reported Claims | claim 1,764                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 579 527 494 414 315 227 125 1    
U.S. Excess Casualty | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 584 604 600 612 597 583 577      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 225                  
Cumulative Number of Reported Claims | claim 1,588                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 282 253 216 157 79 43 7      
U.S. Excess Casualty | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 395 404 420 410 413 406        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 166                  
Cumulative Number of Reported Claims | claim 1,570                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 209 188 128 33 15 4        
U.S. Excess Casualty | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 383 358 274 277 278          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 113                  
Cumulative Number of Reported Claims | claim 1,196                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 207 161 62 43 4          
U.S. Excess Casualty | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 369 333 305 305            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 104                  
Cumulative Number of Reported Claims | claim 824                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 177 96 51 14            
U.S. Excess Casualty | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 373 348 345              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 179                  
Cumulative Number of Reported Claims | claim 740                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 115 89 1              
U.S. Excess Casualty | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 315 315                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 214                  
Cumulative Number of Reported Claims | claim 446                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 59 0                
U.S. Excess Casualty | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 459                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 451                  
Cumulative Number of Reported Claims | claim 185                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 5                  
U.S. Excess Casualty | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 455                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (2)                  
U.S. Excess Casualty | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 2,698                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 70                  
U.S. Excess Casualty | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 3,153                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 68                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 14                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 82                  
U.S. Other Casualty                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 8,691                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 4,329                  
Year 1 (as a percent) 5.30%                  
Year 2 (as a percent) 9.60%                  
Year 3 (as a percent) 11.90%                  
Year 4 (as a percent) 13.20%                  
Year 5 (as a percent) 12.60%                  
Year 6 (as a percent) 9.20%                  
Year 7 (as a percent) 7.10%                  
Year 8 (as a percent) 4.50%                  
Year 9 (as a percent) 2.90%                  
Year 10 (as a percent) 2.30%                  
U.S. Other Casualty | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 1,241 1,264 1,297 1,293 1,323 1,340 1,391 1,321 1,343 1,339
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 35                  
Cumulative Number of Reported Claims | claim 29,467                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,173 1,144 1,074 1,018 938 846 703 489 298 77
U.S. Other Casualty | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 654 662 654 643 668 674 738 629 602  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 21                  
Cumulative Number of Reported Claims | claim 21,436                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 615 613 592 527 455 314 216 111 51  
U.S. Other Casualty | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 852 841 810 824 870 837 845 802    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 115                  
Cumulative Number of Reported Claims | claim 17,192                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 685 636 565 470 360 227 122 43    
U.S. Other Casualty | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,009 1,024 1,039 1,062 1,053 1,058 1,059      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 432                  
Cumulative Number of Reported Claims | claim 21,479                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 510 474 410 321 226 138 53      
U.S. Other Casualty | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 499 519 540 538 576 524        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 166                  
Cumulative Number of Reported Claims | claim 11,792                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 301 252 198 139 73 26        
U.S. Other Casualty | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 845 818 790 793 795          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 461                  
Cumulative Number of Reported Claims | claim 11,552                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 351 265 169 87 32          
U.S. Other Casualty | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 861 827 819 793            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 495                  
Cumulative Number of Reported Claims | claim 14,193                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 299 195 112 38            
U.S. Other Casualty | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 983 955 933              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 698                  
Cumulative Number of Reported Claims | claim 14,983                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 205 102 36              
U.S. Other Casualty | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 871 840                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 709                  
Cumulative Number of Reported Claims | claim 11,999                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 128 37                
U.S. Other Casualty | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 876                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 734                  
Cumulative Number of Reported Claims | claim 7,487                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 62                  
U.S. Other Casualty | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 289                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (62)                  
U.S. Other Casualty | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 4,362                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 65                  
U.S. Other Casualty | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 4,651                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 3                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 19                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 22                  
U.S. Financial Lines                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 15,342                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 10,100                  
Year 1 (as a percent) 4.00%                  
Year 2 (as a percent) 14.30%                  
Year 3 (as a percent) 15.70%                  
Year 4 (as a percent) 14.80%                  
Year 5 (as a percent) 12.50%                  
Year 6 (as a percent) 8.00%                  
Year 7 (as a percent) 6.50%                  
Year 8 (as a percent) 5.50%                  
Year 9 (as a percent) 2.30%                  
Year 10 (as a percent) 2.30%                  
U.S. Financial Lines | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 2,345 2,322 2,308 2,325 2,281 2,139 2,064 1,993 1,855 1,605
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 95                  
Cumulative Number of Reported Claims | claim 16,143                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,169 2,115 2,039 1,903 1,826 1,659 1,358 1,002 499 73
U.S. Financial Lines | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,966 1,969 1,957 1,987 1,898 1,846 1,756 1,675 1,564  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 108                  
Cumulative Number of Reported Claims | claim 15,293                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,805 1,777 1,653 1,515 1,396 1,118 761 391 64  
U.S. Financial Lines | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,260 2,282 2,322 2,225 2,063 1,882 1,766 1,640    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 291                  
Cumulative Number of Reported Claims | claim 14,867                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,879 1,776 1,601 1,415 1,126 835 486 86    
U.S. Financial Lines | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,961 1,945 1,912 1,926 1,627 1,536 1,503      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 266                  
Cumulative Number of Reported Claims | claim 13,406                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,575 1,423 1,204 953 642 367 94      
U.S. Financial Lines | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,480 1,470 1,457 1,408 1,252 1,213        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 162                  
Cumulative Number of Reported Claims | claim 10,495                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,171 1,063 915 648 356 84        
U.S. Financial Lines | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,244 1,316 1,388 1,408 1,430          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 514                  
Cumulative Number of Reported Claims | claim 7,280                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 628 468 315 151 43          
U.S. Financial Lines | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,057 1,105 1,108 1,130            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 695                  
Cumulative Number of Reported Claims | claim 5,985                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 293 177 109 30            
U.S. Financial Lines | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,075 1,041 1,043              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 574                  
Cumulative Number of Reported Claims | claim 7,100                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 324 150 46              
U.S. Financial Lines | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 955 942                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 636                  
Cumulative Number of Reported Claims | claim 7,791                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 216 44                
U.S. Financial Lines | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 999                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 927                  
Cumulative Number of Reported Claims | claim 8,079                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 40                  
U.S. Financial Lines | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 28                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 20                  
U.S. Financial Lines | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 5,242                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (49)                  
U.S. Financial Lines | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 5,270                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (29)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (16)                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (45)                  
U.S. Property and Special Risks                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 27,208                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 23,320                  
Year 1 (as a percent) 36.40%                  
Year 2 (as a percent) 30.10%                  
Year 3 (as a percent) 10.40%                  
Year 4 (as a percent) 7.10%                  
Year 5 (as a percent) 4.70%                  
Year 6 (as a percent) 3.70%                  
Year 7 (as a percent) 2.40%                  
Year 8 (as a percent) 1.50%                  
Year 9 (as a percent) 0.90%                  
Year 10 (as a percent) 0.30%                  
U.S. Property and Special Risks | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 2,731 2,727 2,713 2,700 2,694 2,707 2,697 2,690 2,748 2,674
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 6                  
Cumulative Number of Reported Claims | claim 54,963                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,688 2,679 2,647 2,616 2,539 2,464 2,296 2,076 1,747 821
U.S. Property and Special Risks | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 4,178 4,187 4,175 4,212 4,173 4,153 4,127 4,239 4,569  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 6                  
Cumulative Number of Reported Claims | claim 79,982                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 4,151 4,129 4,055 3,999 3,897 3,638 3,281 2,625 1,137  
U.S. Property and Special Risks | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 3,203 3,204 3,210 3,201 3,229 2,992 2,993 2,978    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 10                  
Cumulative Number of Reported Claims | claim 70,274                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 3,144 3,094 2,994 2,863 2,715 2,509 2,162 977    
U.S. Property and Special Risks | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,215 2,198 2,177 2,222 2,211 2,146 2,177      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 24                  
Cumulative Number of Reported Claims | claim 79,150                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,176 2,124 2,083 2,037 1,906 1,673 1,039      
U.S. Property and Special Risks | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 3,289 3,269 3,238 3,280 3,320 3,391        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 464                  
Cumulative Number of Reported Claims | claim 69,329                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,657 2,414 2,190 1,874 1,613 844        
U.S. Property and Special Risks | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,180 2,190 2,160 2,213 2,339          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 92                  
Cumulative Number of Reported Claims | claim 82,535                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,061 2,004 1,983 1,743 878          
U.S. Property and Special Risks | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 3,234 3,240 3,281 3,171            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 249                  
Cumulative Number of Reported Claims | claim 87,066                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,769 2,437 2,207 1,208            
U.S. Property and Special Risks | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,367 2,440 2,528              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 326                  
Cumulative Number of Reported Claims | claim 100,257                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,878 1,692 1,173              
U.S. Property and Special Risks | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,714 1,844                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 426                  
Cumulative Number of Reported Claims | claim 31,463                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,012 667                
U.S. Property and Special Risks | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,097                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 739                  
Cumulative Number of Reported Claims | claim 22,479                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 784                  
U.S. Property and Special Risks | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 254                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 34                  
U.S. Property and Special Risks | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 3,888                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (188)                  
U.S. Property and Special Risks | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 4,142                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (154)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 4                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (150)                  
U.S. Personal Insurance                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 10,796                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 10,050                  
Year 1 (as a percent) 57.40%                  
Year 2 (as a percent) 24.50%                  
Year 3 (as a percent) 4.90%                  
Year 4 (as a percent) 1.80%                  
Year 5 (as a percent) 2.60%                  
Year 6 (as a percent) 1.00%                  
Year 7 (as a percent) 0.70%                  
Year 8 (as a percent) 0.70%                  
Year 9 (as a percent) 0.20%                  
Year 10 (as a percent) 0.10%                  
U.S. Personal Insurance | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 1,540 1,541 1,541 1,544 1,544 1,542 1,540 1,533 1,533 1,536
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 16                  
Cumulative Number of Reported Claims | claim 247,862                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,522 1,521 1,521 1,518 1,512 1,501 1,460 1,422 1,344 857
U.S. Personal Insurance | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,898 1,899 1,896 1,916 1,924 2,057 2,011 2,137 1,878  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 12                  
Cumulative Number of Reported Claims | claim 220,439                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,884 1,878 1,861 1,852 1,826 1,789 1,896 1,672 941  
U.S. Personal Insurance | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,924 1,927 1,920 1,936 1,937 2,154 2,193 2,188    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 24                  
Cumulative Number of Reported Claims | claim 102,585                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,900 1,881 1,849 1,832 1,789 1,973 1,939 1,227    
U.S. Personal Insurance | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,565 1,569 1,578 1,596 1,646 1,664 1,593      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 45                  
Cumulative Number of Reported Claims | claim 94,184                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,517 1,516 1,491 1,416 1,379 1,295 884      
U.S. Personal Insurance | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 888 890 894 913 906 954        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 37                  
Cumulative Number of Reported Claims | claim 56,199                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 850 846 824 725 679 667        
U.S. Personal Insurance | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 747 752 762 765 748          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 53                  
Cumulative Number of Reported Claims | claim 57,895                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 676 662 658 650 488          
U.S. Personal Insurance | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 523 525 529 517            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 51                  
Cumulative Number of Reported Claims | claim 56,476                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 464 406 401 372            
U.S. Personal Insurance | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 688 668 677              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 53                  
Cumulative Number of Reported Claims | claim 50,195                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 579 522 400              
U.S. Personal Insurance | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 594 604                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 67                  
Cumulative Number of Reported Claims | claim 41,541                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 516 273                
U.S. Personal Insurance | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 429                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 216                  
Cumulative Number of Reported Claims | claim 23,548                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 142                  
U.S. Personal Insurance | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance (41)                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (1)                  
U.S. Personal Insurance | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 746                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (8)                  
U.S. Personal Insurance | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 705                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (9)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (2)                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (11)                  
UK/Europe Casualty and Financial lines                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 14,600                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 7,339                  
Year 1 (as a percent) 5.30%                  
Year 2 (as a percent) 13.40%                  
Year 3 (as a percent) 10.90%                  
Year 4 (as a percent) 10.30%                  
Year 5 (as a percent) 9.00%                  
Year 6 (as a percent) 9.10%                  
Year 7 (as a percent) 5.70%                  
Year 8 (as a percent) 5.70%                  
Year 9 (as a percent) 3.30%                  
Year 10 (as a percent) 3.20%                  
UK/Europe Casualty and Financial lines | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 1,819 1,766 1,687 1,686 1,674 1,685 1,566 1,561 1,482 1,392
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 194                  
Cumulative Number of Reported Claims | claim 142,980                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,418 1,359 1,316 1,221 1,108 975 812 616 399 125
UK/Europe Casualty and Financial lines | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,533 1,520 1,473 1,434 1,449 1,418 1,310 1,349 1,423  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 162                  
Cumulative Number of Reported Claims | claim 150,034                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,180 1,115 1,007 938 789 631 471 295 102  
UK/Europe Casualty and Financial lines | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,849 1,831 1,742 1,696 1,600 1,569 1,465 1,413    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 263                  
Cumulative Number of Reported Claims | claim 151,989                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,365 1,275 1,179 944 778 599 392 120    
UK/Europe Casualty and Financial lines | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,480 1,417 1,407 1,420 1,404 1,554 1,297      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 243                  
Cumulative Number of Reported Claims | claim 143,228                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,023 920 797 691 504 325 103      
UK/Europe Casualty and Financial lines | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,180 1,215 1,266 1,278 1,338 1,319        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 252                  
Cumulative Number of Reported Claims | claim 86,594                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 720 630 533 385 239 63        
UK/Europe Casualty and Financial lines | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,304 1,357 1,398 1,404 1,443          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 458                  
Cumulative Number of Reported Claims | claim 78,019                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 610 474 361 244 54          
UK/Europe Casualty and Financial lines | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,293 1,279 1,320 1,370            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 621                  
Cumulative Number of Reported Claims | claim 75,486                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 404 310 195 60            
UK/Europe Casualty and Financial lines | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,364 1,337 1,361              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 741                  
Cumulative Number of Reported Claims | claim 71,761                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 335 200 38              
UK/Europe Casualty and Financial lines | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,428 1,389                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 919                  
Cumulative Number of Reported Claims | claim 73,433                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 222 63                
UK/Europe Casualty and Financial lines | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,350                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 1,132                  
Cumulative Number of Reported Claims | claim 61,557                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 62                  
UK/Europe Casualty and Financial lines | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 1,027                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 7                  
UK/Europe Casualty and Financial lines | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 7,261                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 139                  
UK/Europe Casualty and Financial lines | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 8,288                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 146                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 70                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 216                  
UK/Europe Property and Special Risks                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 12,971                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 10,885                  
Year 1 (as a percent) 20.10%                  
Year 2 (as a percent) 36.70%                  
Year 3 (as a percent) 18.70%                  
Year 4 (as a percent) 15.00%                  
Year 5 (as a percent) 4.00%                  
Year 6 (as a percent) 3.20%                  
Year 7 (as a percent) 0.80%                  
Year 8 (as a percent) 0.00%                  
Year 9 (as a percent) 0.40%                  
Year 10 (as a percent) (0.30%)                  
UK/Europe Property and Special Risks | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 1,629 1,651 1,659 1,663 1,727 1,733 1,737 1,762 1,740 1,645
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 17                  
Cumulative Number of Reported Claims | claim 57,394                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,616 1,621 1,618 1,643 1,625 1,596 1,550 1,412 1,154 472
UK/Europe Property and Special Risks | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,601 1,604 1,603 1,605 1,639 1,654 1,648 1,674 1,671  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ (2)                  
Cumulative Number of Reported Claims | claim 53,611                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,529 1,518 1,512 1,497 1,458 1,399 1,255 968 363  
UK/Europe Property and Special Risks | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,491 1,500 1,514 1,514 1,567 1,580 1,605 1,678    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 7                  
Cumulative Number of Reported Claims | claim 44,446                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,424 1,407 1,382 1,347 1,331 1,199 1,008 326    
UK/Europe Property and Special Risks | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,112 1,130 1,126 1,122 1,132 1,133 1,172      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 54                  
Cumulative Number of Reported Claims | claim 34,054                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,007 1,013 981 935 846 672 273      
UK/Europe Property and Special Risks | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,224 1,242 1,247 1,224 1,264 1,324        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ (2)                  
Cumulative Number of Reported Claims | claim 26,576                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,131 1,048 934 836 683 254        
UK/Europe Property and Special Risks | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 972 987 962 1,011 1,048          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 69                  
Cumulative Number of Reported Claims | claim 23,384                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 809 779 711 519 194          
UK/Europe Property and Special Risks | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,288 1,282 1,308 1,150            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 272                  
Cumulative Number of Reported Claims | claim 25,559                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,866 1,145 684 198            
UK/Europe Property and Special Risks | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,060 1,086 1,090              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 135                  
Cumulative Number of Reported Claims | claim 24,666                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 744 537 161              
UK/Europe Property and Special Risks | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,260 1,177                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 189                  
Cumulative Number of Reported Claims | claim 24,338                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 554 206                
UK/Europe Property and Special Risks | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,334                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 585                  
Cumulative Number of Reported Claims | claim 17,987                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 205                  
UK/Europe Property and Special Risks | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 90                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (9)                  
UK/Europe Property and Special Risks | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 2,086                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (22)                  
UK/Europe Property and Special Risks | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 2,176                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (31)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 17                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years (14)                  
UK/Europe and Japan Personal Insurance                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 17,828                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 16,632                  
Year 1 (as a percent) 59.00%                  
Year 2 (as a percent) 25.90%                  
Year 3 (as a percent) 6.90%                  
Year 4 (as a percent) 3.30%                  
Year 5 (as a percent) 1.70%                  
Year 6 (as a percent) 1.30%                  
Year 7 (as a percent) 0.60%                  
Year 8 (as a percent) 0.30%                  
Year 9 (as a percent) 0.30%                  
Year 10 (as a percent) 0.00%                  
UK/Europe and Japan Personal Insurance | 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance $ 2,128 2,127 2,131 2,130 2,137 2,140 2,143 2,147 2,140 2,146
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 2                  
Cumulative Number of Reported Claims | claim 1,794,526                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,117 2,116 2,111 2,106 2,091 2,066 2,021 1,935 1,760 $ 1,175
UK/Europe and Japan Personal Insurance | 2017                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,032 2,028 2,032 2,022 2,039 2,022 2,027 2,041 2,064  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 5                  
Cumulative Number of Reported Claims | claim 1,719,905                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,010 2,003 1,993 1,995 1,974 1,939 1,871 1,718 $ 1,153  
UK/Europe and Japan Personal Insurance | 2018                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 2,305 2,306 2,317 2,317 2,295 2,323 2,329 2,423    
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ (1)                  
Cumulative Number of Reported Claims | claim 1,918,168                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 2,293 2,286 2,267 2,219 2,178 2,099 1,946 $ 1,448    
UK/Europe and Japan Personal Insurance | 2019                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,884 1,885 1,879 1,888 1,899 1,936 1,981      
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 2                  
Cumulative Number of Reported Claims | claim 1,677,776                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,864 1,849 1,828 1,806 1,746 1,628 $ 1,156      
UK/Europe and Japan Personal Insurance | 2020                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,603 1,603 1,613 1,627 1,683 1,815        
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 2                  
Cumulative Number of Reported Claims | claim 1,394,864                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,576 1,560 1,533 1,496 1,396 $ 969        
UK/Europe and Japan Personal Insurance | 2021                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,612 1,611 1,613 1,638 1,687          
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 11                  
Cumulative Number of Reported Claims | claim 1,396,679                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,543 1,518 1,457 1,352 $ 952          
UK/Europe and Japan Personal Insurance | 2022                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,742 1,747 1,809 1,788            
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 19                  
Cumulative Number of Reported Claims | claim 2,079,335                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,654 1,600 1,485 $ 1,038            
UK/Europe and Japan Personal Insurance | 2023                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,566 1,578 1,607              
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 36                  
Cumulative Number of Reported Claims | claim 1,456,046                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,437 1,329 $ 906              
UK/Europe and Japan Personal Insurance | 2024                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,511 1,514                
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 41                  
Cumulative Number of Reported Claims | claim 1,422,120                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,278 $ 872                
UK/Europe and Japan Personal Insurance | 2025                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Incurred losses and allocated loss adjustment expenses, undiscounted and net of reinsurance 1,445                  
Total of IBNR Liabilities Plus Expected Development on Reported Losses $ 217                  
Cumulative Number of Reported Claims | claim 1,221,216                  
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance $ 860                  
UK/Europe and Japan Personal Insurance | Accident Years prior to 2016                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 44                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance 0                  
UK/Europe and Japan Personal Insurance | Short Duration Insurance Contracts Last Ten Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 1,196                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (16)                  
UK/Europe and Japan Personal Insurance | All Accident Years                    
Liability for Claims and Claims Adjustment Expense [Line Items]                    
Liabilities for losses and loss adjustment expenses and prior year development before accident year 2016, net of reinsurance 1,240                  
Liabilities for losses and loss adjustment expenses and prior year loss development, net of reinsurance (16)                  
Prior Year Development for Unallocated Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years 53                  
Prior Year Development for Loss and Loss Adjustment Expense, Undiscounted and Net of Reinsurance including ADC, All Accident Years $ 37