Investments - Unrealized Capital Losses (Details) $ in Millions	Jun. 30, 2020 USD ($) securities	Dec. 31, 2019 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 3,839	$ 1,389
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 239	$ 17
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	749	289
More Than Twelve Months Below Amortized Cost, Fair Value	$ 605	$ 1,043
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 60	$ 46
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	190	278
Total, Fair Value	$ 4,444	$ 2,432
Total Unrealized Capital Losses	$ 299	$ 63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	939	567
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 0	$ 68
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 0	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	0
More Than Twelve Months Below Amortized Cost, Fair Value	$ 0	12
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 0	0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	0
Total, Fair Value	$ 0	80
Total Unrealized Capital Losses	$ 0	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 21	21
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 0	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	10
More Than Twelve Months Below Amortized Cost, Fair Value	$ 0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 0	0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	0
Total, Fair Value	$ 21	21
Total Unrealized Capital Losses	$ 0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	10
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 478	97
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 29	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	90
More Than Twelve Months Below Amortized Cost, Fair Value	$ 41	131
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 9	10
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	12
Total, Fair Value	$ 519	228
Total Unrealized Capital Losses	$ 38	13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	102
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 297	75
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 10	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	31
More Than Twelve Months Below Amortized Cost, Fair Value	$ 79	134
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 21	16
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	9
Total, Fair Value	$ 376	209
Total Unrealized Capital Losses	$ 31	16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	40
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 135	6
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 8	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	38
More Than Twelve Months Below Amortized Cost, Fair Value	$ 17	53
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 3	3
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	6
Total, Fair Value	$ 152	59
Total Unrealized Capital Losses	$ 11	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	44
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 314	21
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 28	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	31
More Than Twelve Months Below Amortized Cost, Fair Value	$ 46	56
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 1	1
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	6
Total, Fair Value	$ 360	77
Total Unrealized Capital Losses	$ 29	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	37
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 711	535
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 28	6
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	142
More Than Twelve Months Below Amortized Cost, Fair Value	$ 135	139
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 8	5
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	54
Total, Fair Value	$ 846	674
Total Unrealized Capital Losses	$ 36	11
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	196
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,119	331
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 115	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	224
More Than Twelve Months Below Amortized Cost, Fair Value	$ 9	18
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 1	0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	1
Total, Fair Value	$ 1,128	349
Total Unrealized Capital Losses	$ 116	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	225
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 764	217
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 21	2
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	183
More Than Twelve Months Below Amortized Cost, Fair Value	$ 278	500
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 17	11
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	102
Total, Fair Value	$ 1,042	717
Total Unrealized Capital Losses	$ 38	$ 13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	285