Investments - Debt Securities (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 15,708,000,000	$ 15,119,000,000
Gross Unrealized Gains	1,063,000,000	172,000,000
Gross Unrealized Losses	(13,000,000)	(36,000,000)
Fair Value	16,758,000,000	15,255,000,000
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,743,000,000	1,662,000,000
Gross Unrealized Gains	83,000,000	26,000,000
Gross Unrealized Losses	0	0
Fair Value	1,826,000,000	1,688,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,240,000,000	2,370,000,000
Gross Unrealized Gains	114,000,000	30,000,000
Gross Unrealized Losses	(1,000,000)	(1,000,000)
Fair Value	2,353,000,000	2,399,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,076,000,000	6,444,000,000
Gross Unrealized Gains	568,000,000	61,000,000
Gross Unrealized Losses	(7,000,000)	(16,000,000)
Fair Value	7,637,000,000	6,489,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,095,000,000	2,355,000,000
Gross Unrealized Gains	194,000,000	31,000,000
Gross Unrealized Losses	0	(3,000,000)
Fair Value	2,289,000,000	2,383,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	529,000,000	567,000,000
Gross Unrealized Gains	25,000,000	10,000,000
Gross Unrealized Losses	0	0
Fair Value	554,000,000	577,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	645,000,000	594,000,000
Gross Unrealized Gains	56,000,000	11,000,000
Gross Unrealized Losses	0	0
Fair Value	701,000,000	605,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,348,000,000	1,097,000,000
Gross Unrealized Gains	15,000,000	3,000,000
Gross Unrealized Losses	(5,000,000)	(15,000,000)
Fair Value	1,358,000,000	1,085,000,000
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,000,000	30,000,000
Gross Unrealized Gains	8,000,000	0
Gross Unrealized Losses	0	(1,000,000)
Fair Value	40,000,000	29,000,000
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	85,000,000	12,000,000
Gross Unrealized Losses	0	(2,000,000)
Fair Value	$ 973,000,000	$ 916,000,000