Investments - Debt Securities (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	$ 16,145,000,000	$ 15,898,000,000
Allowance for Credit Losses	(48,000,000)	0
Net Amortized Cost	16,097,000,000	15,898,000,000
Gross Unrealized Gains	866,000,000	1,035,000,000
Gross Unrealized Losses	(246,000,000)	(11,000,000)
Fair Value	16,717,000,000	16,922,000,000
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	1,853,000,000	1,791,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	1,853,000,000	1,791,000,000
Gross Unrealized Gains	162,000,000	62,000,000
Gross Unrealized Losses	0	(1,000,000)
Fair Value	2,015,000,000	1,852,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,182,000,000	2,202,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,182,000,000	2,202,000,000
Gross Unrealized Gains	91,000,000	108,000,000
Gross Unrealized Losses	(7,000,000)	(1,000,000)
Fair Value	2,266,000,000	2,309,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	7,128,000,000	7,167,000,000
Allowance for Credit Losses	(28,000,000)	0
Net Amortized Cost	7,100,000,000	7,167,000,000
Gross Unrealized Gains	412,000,000	573,000,000
Gross Unrealized Losses	(109,000,000)	(3,000,000)
Fair Value	7,403,000,000	7,737,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,202,000,000	2,149,000,000
Allowance for Credit Losses	(18,000,000)	0
Net Amortized Cost	2,184,000,000	2,149,000,000
Gross Unrealized Gains	112,000,000	200,000,000
Gross Unrealized Losses	(48,000,000)	(1,000,000)
Fair Value	2,248,000,000	2,348,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	574,000,000	508,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	574,000,000	508,000,000
Gross Unrealized Gains	38,000,000	25,000,000
Gross Unrealized Losses	0	0
Fair Value	612,000,000	533,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	662,000,000	654,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	662,000,000	654,000,000
Gross Unrealized Gains	43,000,000	46,000,000
Gross Unrealized Losses	(1,000,000)	0
Fair Value	704,000,000	700,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	1,509,000,000	1,397,000,000
Allowance for Credit Losses	(2,000,000)	0
Net Amortized Cost	1,507,000,000	1,397,000,000
Gross Unrealized Gains	7,000,000	13,000,000
Gross Unrealized Losses	(80,000,000)	(5,000,000)
Fair Value	1,434,000,000	1,405,000,000
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	35,000,000	30,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	35,000,000	30,000,000
Gross Unrealized Gains	1,000,000	8,000,000
Gross Unrealized Losses	(1,000,000)	0
Fair Value	35,000,000	38,000,000
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	65,000,000	83,000,000
Gross Unrealized Losses	(10,000,000)	0
Fair Value	$ 917,000,000	$ 965,000,000