Investments - Debt Securities (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 19,396,000,000	$ 15,898,000,000
Gross Unrealized Gains	1,803,000,000	1,035,000,000
Gross Unrealized Losses	(11,000,000)	(11,000,000)
Fair Value	21,188,000,000	16,922,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,341,000,000	1,791,000,000
Gross Unrealized Gains	128,000,000	62,000,000
Gross Unrealized Losses	0	(1,000,000)
Fair Value	2,469,000,000	1,852,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,556,000,000	2,202,000,000
Gross Unrealized Gains	172,000,000	108,000,000
Gross Unrealized Losses	0	(1,000,000)
Fair Value	2,728,000,000	2,309,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,879,000,000	7,167,000,000
Gross Unrealized Gains	1,023,000,000	573,000,000
Gross Unrealized Losses	(8,000,000)	(3,000,000)
Fair Value	8,894,000,000	7,737,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,595,000,000	2,149,000,000
Gross Unrealized Gains	324,000,000	200,000,000
Gross Unrealized Losses	(1,000,000)	(1,000,000)
Fair Value	2,918,000,000	2,348,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	673,000,000	508,000,000
Gross Unrealized Gains	32,000,000	25,000,000
Gross Unrealized Losses	0	0
Fair Value	705,000,000	533,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	962,000,000	654,000,000
Gross Unrealized Gains	84,000,000	46,000,000
Gross Unrealized Losses	0	0
Fair Value	1,046,000,000	700,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,369,000,000	1,397,000,000
Gross Unrealized Gains	36,000,000	13,000,000
Gross Unrealized Losses	(2,000,000)	(5,000,000)
Fair Value	2,403,000,000	1,405,000,000
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,000,000	30,000,000
Gross Unrealized Gains	4,000,000	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	135,000,000	83,000,000
Gross Unrealized Losses	0	0
Fair Value	$ 919,000,000	$ 965,000,000