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Acquisitions and Divestitures - Willis Re Divestiture (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended	9 Months Ended
	Oct. 31, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Jun. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 01, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash outflow of cash and non-fiduciary cash related to settlement						$ 0	$ 922
Gain on disposal						(1,190)	44
Willis Re [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total consideration received from sale of subsidiary								$ 3,250
Gain on disposal			$ 5
Value of remaining settlement amount	$ 11
Willis Re [Member] \| Transition Services Agreement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fees earned under service agreement		$ 7	$ 11			18	$ 29
Willis Re [Member] \| Maximum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Earnout receivable in cash in 2025								$ 750
Willis Re [Member] \| Gallagher [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Value of obligation in initial transfer				$ 74	$ 74
Value of receivables in initial transfer				26	26
Cash outflow of cash and non-fiduciary cash related to settlement					$ 916
Fiduciary assets transferred				4,500
Willis Re [Member] \| Gallagher [Member] \| Non-Fiduciary Cash [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash outflow of cash and non-fiduciary cash related to settlement				48
Willis Re [Member] \| Gallagher [Member] \| Fiduciary Cash [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash outflow of cash and non-fiduciary cash related to settlement				$ 868
TRANZACT [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total consideration received from sale of subsidiary		$ 632				$ 632