v3.26.1
Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Assets:    
Total fixed maturity securities available for sale $ 25,884,009 $ 25,017,056
Equity securities 1,502,237 1,358,201
Arbitrage trading account 1,292,382 1,221,103
Fair Value, Measurements, Recurring    
Assets:    
Total fixed maturity securities available for sale 25,884,009 25,017,056
Equity securities 1,502,237 1,358,201
Arbitrage trading account 1,292,382 1,221,103
Total 28,678,628 27,596,360
Liabilities:    
Trading account securities sold but not yet purchased 6,576 66,285
Fair Value, Measurements, Recurring | U.S. government and government agency    
Assets:    
Total fixed maturity securities available for sale 4,402,996 3,998,038
Fair Value, Measurements, Recurring | State and municipal    
Assets:    
Total fixed maturity securities available for sale 1,642,356 1,837,194
Fair Value, Measurements, Recurring | Mortgage-backed    
Assets:    
Total fixed maturity securities available for sale 4,483,852 4,808,478
Fair Value, Measurements, Recurring | Asset-backed    
Assets:    
Total fixed maturity securities available for sale 4,076,528 3,810,346
Fair Value, Measurements, Recurring | Corporate    
Assets:    
Total fixed maturity securities available for sale 9,278,095 8,687,411
Fair Value, Measurements, Recurring | Foreign government    
Assets:    
Total fixed maturity securities available for sale 2,000,182 1,875,589
Fair Value, Measurements, Recurring | Common stocks    
Assets:    
Equity securities 883,940 742,113
Fair Value, Measurements, Recurring | Preferred stocks    
Assets:    
Equity securities 618,297 616,088
Level 1 | Fair Value, Measurements, Recurring    
Assets:    
Total fixed maturity securities available for sale 0 0
Equity securities 880,946 739,186
Arbitrage trading account 1,199,119 1,139,447
Total 2,080,065 1,878,633
Liabilities:    
Trading account securities sold but not yet purchased 6,576 66,285
Level 1 | Fair Value, Measurements, Recurring | U.S. government and government agency    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | State and municipal    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | Mortgage-backed    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | Asset-backed    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | Corporate    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | Foreign government    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 1 | Fair Value, Measurements, Recurring | Common stocks    
Assets:    
Equity securities 880,946 739,186
Level 1 | Fair Value, Measurements, Recurring | Preferred stocks    
Assets:    
Equity securities 0 0
Level 2 | Fair Value, Measurements, Recurring    
Assets:    
Total fixed maturity securities available for sale 25,864,225 24,997,055
Equity securities 610,547 608,200
Arbitrage trading account 93,150 81,543
Total 26,567,922 25,686,798
Liabilities:    
Trading account securities sold but not yet purchased 0 0
Level 2 | Fair Value, Measurements, Recurring | U.S. government and government agency    
Assets:    
Total fixed maturity securities available for sale 4,402,996 3,998,038
Level 2 | Fair Value, Measurements, Recurring | State and municipal    
Assets:    
Total fixed maturity securities available for sale 1,642,356 1,837,194
Level 2 | Fair Value, Measurements, Recurring | Mortgage-backed    
Assets:    
Total fixed maturity securities available for sale 4,483,852 4,808,478
Level 2 | Fair Value, Measurements, Recurring | Asset-backed    
Assets:    
Total fixed maturity securities available for sale 4,076,528 3,810,346
Level 2 | Fair Value, Measurements, Recurring | Corporate    
Assets:    
Total fixed maturity securities available for sale 9,258,311 8,667,410
Level 2 | Fair Value, Measurements, Recurring | Foreign government    
Assets:    
Total fixed maturity securities available for sale 2,000,182 1,875,589
Level 2 | Fair Value, Measurements, Recurring | Common stocks    
Assets:    
Equity securities 925 786
Level 2 | Fair Value, Measurements, Recurring | Preferred stocks    
Assets:    
Equity securities 609,622 607,414
Level 3 | Fair Value, Measurements, Recurring    
Assets:    
Total fixed maturity securities available for sale 19,784 20,001
Equity securities 10,744 10,815
Arbitrage trading account 113 113
Total 30,641 30,929
Liabilities:    
Trading account securities sold but not yet purchased 0 0
Level 3 | Fair Value, Measurements, Recurring | U.S. government and government agency    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 3 | Fair Value, Measurements, Recurring | State and municipal    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 3 | Fair Value, Measurements, Recurring | Mortgage-backed    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 3 | Fair Value, Measurements, Recurring | Asset-backed    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 3 | Fair Value, Measurements, Recurring | Corporate    
Assets:    
Total fixed maturity securities available for sale 19,784 20,001
Level 3 | Fair Value, Measurements, Recurring | Foreign government    
Assets:    
Total fixed maturity securities available for sale 0 0
Level 3 | Fair Value, Measurements, Recurring | Common stocks    
Assets:    
Equity securities 2,069 2,141
Level 3 | Fair Value, Measurements, Recurring | Preferred stocks    
Assets:    
Equity securities $ 8,675 $ 8,674