Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 7,657	$ 1,859
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	197	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1,348	119
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	81	3
More Than Twelve Months Below Amortized Cost, Fair Value	1,306	1,692
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	128	82
Total, Fair Value	10,311	3,670
Total Unrealized Capital Losses	$ 406	$ 148
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	91.10%	95.40%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 13	$ 18
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	2	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1	0
More Than Twelve Months Below Amortized Cost, Fair Value	12	12
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total, Fair Value	27	30
Total Unrealized Capital Losses	1	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	196	34
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	3	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	27	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1	0
More Than Twelve Months Below Amortized Cost, Fair Value	65	91
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	4	4
Total, Fair Value	288	126
Total Unrealized Capital Losses	8	4
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	2,384	504
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	70	11
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	407	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	26	0
More Than Twelve Months Below Amortized Cost, Fair Value	197	304
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	23	15
Total, Fair Value	2,988	808
Total Unrealized Capital Losses	119	26
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,258	226
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	26	2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	180	46
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	11	2
More Than Twelve Months Below Amortized Cost, Fair Value	443	499
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	53	34
Total, Fair Value	1,881	771
Total Unrealized Capital Losses	90	38
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	994	148
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	32	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	105	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9	0
More Than Twelve Months Below Amortized Cost, Fair Value	77	99
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12	8
Total, Fair Value	1,176	252
Total Unrealized Capital Losses	53	9
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,107	135
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	46	38
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	67	13
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3	0
More Than Twelve Months Below Amortized Cost, Fair Value	153	161
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	16	7
Total, Fair Value	1,327	309
Total Unrealized Capital Losses	65	45
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	423	263
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	7	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	191	26
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	14	1
More Than Twelve Months Below Amortized Cost, Fair Value	313	438
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	17	13
Total, Fair Value	927	727
Total Unrealized Capital Losses	38	19
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	735	436
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	311	19
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	15	0
More Than Twelve Months Below Amortized Cost, Fair Value	33	50
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2	1
Total, Fair Value	1,079	505
Total Unrealized Capital Losses	27	6
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	547	95
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	3	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	58	9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1	0
More Than Twelve Months Below Amortized Cost, Fair Value	13	38
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	0
Total, Fair Value	618	142
Total Unrealized Capital Losses	$ 5	$ 1