Investments - Unrealized Capital Losses (Details) securities in Millions, $ in Millions	Jun. 30, 2019 USD ($) securities	Dec. 31, 2018 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	$ 1,413	$ 9,733
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	22	359
More Than Twelve Months Below Amortized Cost, Fair Value	1,900	2,392
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	88	201
Total, Fair Value	3,313	12,125
Total Unrealized Capital Losses	$ 110	$ 560
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position, Number of Securities | securities	281	1,894
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	404	550
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | securities	685	2,444
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	$ 0	$ 0
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	13	15
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total, Fair Value	13	15
Total Unrealized Capital Losses	0	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	3	191
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	0	3
More Than Twelve Months Below Amortized Cost, Fair Value	20	88
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	5
Total, Fair Value	23	279
Total Unrealized Capital Losses	1	8
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	173	3,060
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	3	131
More Than Twelve Months Below Amortized Cost, Fair Value	393	535
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	29	50
Total, Fair Value	566	3,595
Total Unrealized Capital Losses	32	181
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	162	1,502
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	3	40
More Than Twelve Months Below Amortized Cost, Fair Value	312	579
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	22	66
Total, Fair Value	474	2,081
Total Unrealized Capital Losses	25	106
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	58	1,159
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	1	54
More Than Twelve Months Below Amortized Cost, Fair Value	172	169
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	14	26
Total, Fair Value	230	1,328
Total Unrealized Capital Losses	15	80
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	52	1,504
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	1	77
More Than Twelve Months Below Amortized Cost, Fair Value	306	221
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	8	20
Total, Fair Value	358	1,725
Total Unrealized Capital Losses	9	97
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	468	560
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	8	11
More Than Twelve Months Below Amortized Cost, Fair Value	272	412
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6	21
Total, Fair Value	740	972
Total Unrealized Capital Losses	14	32
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	111	865
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	1	16
More Than Twelve Months Below Amortized Cost, Fair Value	39	312
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	12
Total, Fair Value	150	1,177
Total Unrealized Capital Losses	2	28
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Fair Value	386	892
Available-for-sale Securities, Including Securities Pledged, Continuous Unrealized Loss Position Twelve Months or Less, Accumulated Loss	5	27
More Than Twelve Months Below Amortized Cost, Fair Value	373	61
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	7	1
Total, Fair Value	759	953
Total Unrealized Capital Losses	$ 12	$ 28