Investments - Unrealized Capital Losses (Details) $ in Millions	Sep. 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	$ 3,364	$ 1,389
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 160	$ 17
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	880	289
More Than Twelve Months Below Amortized Cost, Fair Value	$ 567	$ 1,043
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 47	$ 46
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	191	278
Total, Fair Value	$ 3,931	$ 2,432
Total Unrealized Capital Losses	$ 207	$ 63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	1,071	567
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 9	$ 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	3
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	$ 9	80
Total Unrealized Capital Losses	$ 0	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	3
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 11	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	5
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	$ 11	21
Total Unrealized Capital Losses	$ 0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	5
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 673	97
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 25	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	326
More Than Twelve Months Below Amortized Cost, Fair Value	$ 38	131
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 7	10
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	10
Total, Fair Value	$ 711	228
Total Unrealized Capital Losses	$ 32	13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	336
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 251	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	$ 68	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	8
Total, Fair Value	$ 319	209
Total Unrealized Capital Losses	$ 31	16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	39
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	$ 134	6
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 7	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	62
More Than Twelve Months Below Amortized Cost, Fair Value	$ 10	53
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 2	3
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	4
Total, Fair Value	$ 144	59
Total Unrealized Capital Losses	$ 9	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	66
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 246	21
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 31	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	30
More Than Twelve Months Below Amortized Cost, Fair Value	$ 3	56
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 0	1
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	$ 249	77
Total Unrealized Capital Losses	$ 31	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	31
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 579	535
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 24	6
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	121
More Than Twelve Months Below Amortized Cost, Fair Value	$ 139	139
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 5	5
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	62
Total, Fair Value	$ 718	674
Total Unrealized Capital Losses	$ 29	11
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	183
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 917	331
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 56	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	170
More Than Twelve Months Below Amortized Cost, Fair Value	$ 13	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	3
Total, Fair Value	$ 930	349
Total Unrealized Capital Losses	$ 57	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	173
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 544	217
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 7	2
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	132
More Than Twelve Months Below Amortized Cost, Fair Value	$ 296	500
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 11	11
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	103
Total, Fair Value	$ 840	717
Total Unrealized Capital Losses	$ 18	$ 13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	235