Investments - Debt Securities (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	$ 17,111,000,000	$ 15,898,000,000
Allowance for Credit Losses	(3,000,000)	0
Net Amortized Cost	17,108,000,000	15,898,000,000
Gross Unrealized Gains	1,499,000,000	1,035,000,000
Gross Unrealized Losses	(53,000,000)	(11,000,000)
Fair Value	18,554,000,000	16,922,000,000
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	1,924,000,000	1,791,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	1,924,000,000	1,791,000,000
Gross Unrealized Gains	156,000,000	62,000,000
Gross Unrealized Losses	0	(1,000,000)
Fair Value	2,080,000,000	1,852,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,312,000,000	2,202,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	2,312,000,000	2,202,000,000
Gross Unrealized Gains	138,000,000	108,000,000
Gross Unrealized Losses	(2,000,000)	(1,000,000)
Fair Value	2,448,000,000	2,309,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	7,352,000,000	7,167,000,000
Allowance for Credit Losses	(1,000,000)	0
Net Amortized Cost	7,351,000,000	7,167,000,000
Gross Unrealized Gains	837,000,000	573,000,000
Gross Unrealized Losses	(18,000,000)	(3,000,000)
Fair Value	8,170,000,000	7,737,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	2,266,000,000	2,149,000,000
Allowance for Credit Losses	(1,000,000)	0
Net Amortized Cost	2,265,000,000	2,149,000,000
Gross Unrealized Gains	231,000,000	200,000,000
Gross Unrealized Losses	(11,000,000)	(1,000,000)
Fair Value	2,485,000,000	2,348,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	666,000,000	508,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	666,000,000	508,000,000
Gross Unrealized Gains	36,000,000	25,000,000
Gross Unrealized Losses	0	0
Fair Value	702,000,000	533,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	754,000,000	654,000,000
Allowance for Credit Losses	0	0
Net Amortized Cost	754,000,000	654,000,000
Gross Unrealized Gains	69,000,000	46,000,000
Gross Unrealized Losses	0	0
Fair Value	823,000,000	700,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross Amortized Cost	1,815,000,000	1,397,000,000
Allowance for Credit Losses	(1,000,000)	0
Net Amortized Cost	1,814,000,000	1,397,000,000
Gross Unrealized Gains	30,000,000	13,000,000
Gross Unrealized Losses	(22,000,000)	(5,000,000)
Fair Value	1,822,000,000	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	2,000,000	8,000,000
Gross Unrealized Losses	0	0
Fair Value	24,000,000	38,000,000
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	118,000,000	83,000,000
Gross Unrealized Losses	(2,000,000)	0
Fair Value	$ 931,000,000	$ 965,000,000