Principles of Consolidation And Other Matters (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Operating funds related to regulatory requirements or as collateral under captive insurance arrangements	$ 306,000,000			$ 306,000,000		$ 270,000,000
Equity method investments lag period				3 months
Gain (loss) on investment income, net	$ 13,000,000		$ (14,000,000)	$ 43,000,000	$ (47,000,000)
Effective tax rate (as a percent)	24.20%		30.30%	27.10%	26.00%
Charge related to revaluation of deferred tax assets and liabilities		$ 100,000,000
Increase in effective tax rate due to change in U.K. corporate income tax rate, percent				3.10%
Unrecognized tax benefits	$ 101,000,000			$ 101,000,000		$ 98,000,000
Minimum
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Reasonably possible decrease in unrecognized tax benefits	0			0
Maximum
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Reasonably possible decrease in unrecognized tax benefits	$ 33,000,000			$ 33,000,000
Alexander Forbes Group Holdings Limited
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Gain (loss) on investment income, net			$ (23,000,000)