Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,708	$ 1,859
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	87	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	4,372	119
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	179	3
More Than Twelve Months Below Amortized Cost, Fair Value	1,953	1,692
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	180	82
Total, Fair Value	11,033	3,670
Total Unrealized Capital Losses	$ 446	$ 148
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	91.50%	95.40%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 0	$ 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	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	13	12
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total, Fair Value	15	30
Total Unrealized Capital Losses	0	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	217	34
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	111	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	4	0
More Than Twelve Months Below Amortized Cost, Fair Value	80	91
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6	4
Total, Fair Value	408	126
Total Unrealized Capital Losses	14	4
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,238	504
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	20	11
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1,487	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	59	0
More Than Twelve Months Below Amortized Cost, Fair Value	377	304
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	37	15
Total, Fair Value	3,102	808
Total Unrealized Capital Losses	116	26
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	597	226
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	6	2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	821	46
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	27	2
More Than Twelve Months Below Amortized Cost, Fair Value	569	499
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	63	34
Total, Fair Value	1,987	771
Total Unrealized Capital Losses	96	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	459	148
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	632	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	30	0
More Than Twelve Months Below Amortized Cost, Fair Value	109	99
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	13	8
Total, Fair Value	1,200	252
Total Unrealized Capital Losses	51	9
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	757	135
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	33	38
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	576	13
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	24	0
More Than Twelve Months Below Amortized Cost, Fair Value	173	161
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	17	7
Total, Fair Value	1,506	309
Total Unrealized Capital Losses	74	45
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	281	263
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	211	26
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	12	1
More Than Twelve Months Below Amortized Cost, Fair Value	399	438
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	30	13
Total, Fair Value	891	727
Total Unrealized Capital Losses	46	19
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	612	436
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	407	19
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	190	50
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	13	1
Total, Fair Value	1,209	505
Total Unrealized Capital Losses	42	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	4	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	125	9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2	0
More Than Twelve Months Below Amortized Cost, Fair Value	43	38
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	0
Total, Fair Value	715	142
Total Unrealized Capital Losses	$ 7	$ 1