Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Nov. 30, 2021	Dec. 31, 2020
Tax Credit Carryforward [Line Items]
Undistributed earnings of foreign subsidiaries	$ 830
Differences related to non-U.S. operations	586	$ 543
Net increases in valuation allowances	110	1	$ 36
Change in valuation allowance	$ 94	5	2
Net operating loss carryforwards, subject to expiration (as a percent)	10.00%
Net operating loss carryforwards, not subject to expiration (as a percent)	90.00%
Unrecognized tax benefits	$ 122	$ 94	87
Accrued interest and penalties, before any applicable federal benefit	48		$ 45
Maximum
Tax Credit Carryforward [Line Items]
Reasonably possible decrease in unrecognized tax benefits	58
Marsh India
Tax Credit Carryforward [Line Items]
Percentage of interest acquired			92.00%	49.00%	49.00%
State and Local
Tax Credit Carryforward [Line Items]
Differences related to non-U.S. operations	80
Operating loss carryforwards tax benefits	4
Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	1
Non-U.S.
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	$ 363