Acquisitions and Divestitures - Schedule of Selected Financial Information Relates to the Operations of Willis Re (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Adjustment to gain on disposal of Willis Re	$ 0	$ (56)
Income from discontinued operations, net of tax	$ 0	11
Willis Re [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue from discontinued operations		28
Salaries and benefits		6
Other operating expenses		0
Total costs of providing services		6
Other income, net		0
Income from discontinued operations before income taxes		22
Adjustment to gain on disposal of Willis Re		(2)
Provision for income taxes		(3)
Net income payable to Gallagher on Deferred Closing		(6)
Income from discontinued operations, net of tax		$ 11