Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Nov. 30, 2021
Income Tax Disclosure [Abstract]
Undistributed earnings of foreign subsidiaries	$ 730,000,000
Effective tax rate	24.60%	26.70%	27.30%
Net increases in valuation allowances	$ 36,000,000	$ 72,000,000	$ 60,000,000
Increase (decrease) in income tax expense due to adjustments to valuation allowance	$ 2,000,000	14,000,000	0
Net operating loss carryforwards (as a percent)	29.00%
Tax Credit Carryforward [Line Items]
Differences related to non-U.S. operations	$ 624,000,000	569,000,000
Unrecognized tax benefits	87,000,000	90,000,000	75,000,000
Accrued interest and penalties, before any applicable federal benefit	$ 45,000,000	$ 40,000,000	$ 31,000,000
Marsh India
Tax Credit Carryforward [Line Items]
Percentage of interest acquired	92.00%	49.00%		49.00%
Minimum
Tax Credit Carryforward [Line Items]
Reasonably possible decrease in unrecognized tax benefits	$ 0
Maximum
Tax Credit Carryforward [Line Items]
Reasonably possible decrease in unrecognized tax benefits	47,000,000
Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	9,000,000
State and Local
Tax Credit Carryforward [Line Items]
Differences related to non-U.S. operations	70,000,000
Operating loss carryforwards tax benefits	24,000,000
Non-U.S.
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	$ 246,000,000