Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 5,563	$ 1,859
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	106	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	889	119
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	64	3
More Than Twelve Months Below Amortized Cost, Fair Value	1,524	1,692
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	128	82
Total, Fair Value	7,976	3,670
Total Unrealized Capital Losses	$ 298	$ 148
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	92.30%	95.40%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 15	$ 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	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	12	12
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	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	183	34
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	17	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	89	91
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5	4
Total, Fair Value	289	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	1,904	504
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	38	11
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	184	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	13	0
More Than Twelve Months Below Amortized Cost, Fair Value	228	304
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	22	15
Total, Fair Value	2,316	808
Total Unrealized Capital Losses	73	26
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	932	226
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	17	2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	146	46
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	8	2
More Than Twelve Months Below Amortized Cost, Fair Value	475	499
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	53	34
Total, Fair Value	1,553	771
Total Unrealized Capital Losses	78	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	766	148
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	18	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	40	5
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	83	99
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	10	8
Total, Fair Value	889	252
Total Unrealized Capital Losses	31	9
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	711	135
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	13	38
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	70	13
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	21	0
More Than Twelve Months Below Amortized Cost, Fair Value	166	161
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	14	7
Total, Fair Value	947	309
Total Unrealized Capital Losses	48	45
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	334	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	147	26
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	8	1
More Than Twelve Months Below Amortized Cost, Fair Value	418	438
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	21	13
Total, Fair Value	899	727
Total Unrealized Capital Losses	36	19
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	573	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	240	19
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	37	50
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2	1
Total, Fair Value	850	505
Total Unrealized Capital Losses	21	6
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	145	95
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	45	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	16	38
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total, Fair Value	206	142
Total Unrealized Capital Losses	$ 2	$ 1