Investments - Debt Securities (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	$ 18,863,000,000	$ 15,898,000,000
Allowance for Credit Losses	(4,000,000)	0
Net Amortized Cost	18,859,000,000	15,898,000,000
Gross Unrealized Gains	1,577,000,000	1,035,000,000
Gross Unrealized Losses	(38,000,000)	(11,000,000)
Fair Value	20,398,000,000	16,922,000,000
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,455,000,000	1,791,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,455,000,000	1,791,000,000
Gross Unrealized Gains	146,000,000	62,000,000
Gross Unrealized Losses	0	(1,000,000)
Fair Value	2,601,000,000	1,852,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,393,000,000	2,202,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,393,000,000	2,202,000,000
Gross Unrealized Gains	150,000,000	108,000,000
Gross Unrealized Losses	(3,000,000)	(1,000,000)
Fair Value	2,540,000,000	2,309,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	7,665,000,000	7,167,000,000
Allowance for Credit Losses	(4,000,000)	0
Net Amortized Cost	7,661,000,000	7,167,000,000
Gross Unrealized Gains	878,000,000	573,000,000
Gross Unrealized Losses	(15,000,000)	(3,000,000)
Fair Value	8,524,000,000	7,737,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,479,000,000	2,149,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,479,000,000	2,149,000,000
Gross Unrealized Gains	252,000,000	200,000,000
Gross Unrealized Losses	(10,000,000)	(1,000,000)
Fair Value	2,721,000,000	2,348,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	680,000,000	508,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	680,000,000	508,000,000
Gross Unrealized Gains	33,000,000	25,000,000
Gross Unrealized Losses	0	0
Fair Value	713,000,000	533,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	930,000,000	654,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	930,000,000	654,000,000
Gross Unrealized Gains	82,000,000	46,000,000
Gross Unrealized Losses	0	0
Fair Value	1,012,000,000	700,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,239,000,000	1,397,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,239,000,000	1,397,000,000
Gross Unrealized Gains	33,000,000	13,000,000
Gross Unrealized Losses	(10,000,000)	(5,000,000)
Fair Value	2,262,000,000	1,405,000,000
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	22,000,000	30,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	22,000,000	30,000,000
Gross Unrealized Gains	3,000,000	8,000,000
Gross Unrealized Losses	0	0
Fair Value	25,000,000	38,000,000
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	122,000,000	83,000,000
Gross Unrealized Losses	(1,000,000)	0
Fair Value	$ 928,000,000	$ 965,000,000