Investments - Debt and Equity Securities (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 15,588,000,000	$ 15,119,000,000
Available-for-sale Securities, Gross Unrealized Loss	888,000,000	172,000,000
Available-for-sale Securities, Gross Unrealized Gain	7,000,000	36,000,000
Fair Value	16,469,000,000	15,255,000,000
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,675,000,000	1,662,000,000
Available-for-sale Securities, Gross Unrealized Loss	66,000,000	26,000,000
Available-for-sale Securities, Gross Unrealized Gain	0	0
Fair Value	1,741,000,000	1,688,000,000
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,363,000,000	2,370,000,000
Available-for-sale Securities, Gross Unrealized Loss	103,000,000	30,000,000
Available-for-sale Securities, Gross Unrealized Gain	0	1,000,000
Fair Value	2,466,000,000	2,399,000,000
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,929,000,000	6,444,000,000
Available-for-sale Securities, Gross Unrealized Loss	465,000,000	61,000,000
Available-for-sale Securities, Gross Unrealized Gain	2,000,000	16,000,000
Fair Value	7,392,000,000	6,489,000,000
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,188,000,000	2,355,000,000
Available-for-sale Securities, Gross Unrealized Loss	165,000,000	31,000,000
Available-for-sale Securities, Gross Unrealized Gain	1,000,000	3,000,000
Fair Value	2,352,000,000	2,383,000,000
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	542,000,000	567,000,000
Available-for-sale Securities, Gross Unrealized Loss	22,000,000	10,000,000
Available-for-sale Securities, Gross Unrealized Gain	0	0
Fair Value	564,000,000	577,000,000
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	612,000,000	594,000,000
Available-for-sale Securities, Gross Unrealized Loss	46,000,000	11,000,000
Available-for-sale Securities, Gross Unrealized Gain	0	0
Fair Value	658,000,000	605,000,000
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,247,000,000	1,097,000,000
Available-for-sale Securities, Gross Unrealized Loss	14,000,000	3,000,000
Available-for-sale Securities, Gross Unrealized Gain	4,000,000	15,000,000
Fair Value	1,257,000,000	1,085,000,000
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,000,000	30,000,000
Available-for-sale Securities, Gross Unrealized Loss	7,000,000	0
Available-for-sale Securities, Gross Unrealized Gain	0	1,000,000
Fair Value	39,000,000	29,000,000
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Unrealized Loss	0	2,000,000
Available-for-sale Securities, Gross Unrealized Gain	70,000,000	12,000,000
Fair Value	$ 944,000,000	$ 916,000,000