Principles of Consolidation And Other Matters (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Operating funds related to regulatory requirements or as collateral under captive insurance arrangements	$ 193,000,000		$ 193,000,000			$ 186,000,000
Equity method investments lag period			3 months
Gain (loss) on investment income, net	7,000,000	$ (52,000,000)	$ 20,000,000	$ (24,000,000)
Gains on equity securities	4,000,000	25,000,000	10,000,000	43,000,000
Equity method income	3,000,000	$ 4,000,000	10,000,000	$ 14,000,000
Operating lease, liability	2,298,000,000		2,298,000,000
Operating lease, right-of-use asset	$ 1,957,000,000		$ 1,957,000,000			0
Effective tax rate (as a percent)	26.00%	27.50%	27.80%	25.20%
Unrecognized tax benefits	$ 76,000,000		$ 76,000,000			$ 78,000,000
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	$ 7,000,000		$ 7,000,000
Accounting Standards Update 2016-02
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Operating lease, liability					$ 1,900,000,000
Operating lease, right-of-use asset					1,700,000,000
Restatement Adjustment | Accounting Standards Update 2016-02
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Other liabilities					$ 200,000,000
Alexander Forbes Group Holdings Limited
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Equity method investment other than temporary impairment charge		$ 81,000,000		$ 81,000,000