Acquisitions and Divestitures - Schedule of Selected Financial Information Relates to the Operations of Willis Re (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Transaction and transformation	$ 386	$ 181	$ (806)
(Loss)/gain on disposal of Willis Re	43	(59)	2,679
(Loss)/income from discontinued operations, net of tax	$ 0	(40)	2,080
Willis Re [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Revenue from discontinued operations		48	721
Salaries and benefits		14	350
Other operating expenses		10	59
Depreciation and amortization		0	2
Transaction and transformation		0	33
Total costs of providing services		24	444
Other income, net		5	2
Income from discontinued operations before income taxes		29	279
(Loss)/gain on disposal of Willis Re		(65)	2,300
Benefit from/(provision for) income tax expense		1	(500)
Net income (payable to)/receivable from Gallagher on Deferred Closing		(5)	1
(Loss)/income from discontinued operations, net of tax		$ (40)	$ 2,080