Investments - Unrealized Capital Losses (Details) $ in Millions	Mar. 31, 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	$ 8,404	$ 1,389
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 901	$ 17
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	1,687	289
More Than Twelve Months Below Amortized Cost, Fair Value	$ 505	$ 1,043
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 113	$ 46
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	189	278
Total, Fair Value	$ 8,909	$ 2,432
Total Unrealized Capital Losses	$ 1,014	$ 63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	1,876	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	$ 42	21
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 2	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	15
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	$ 42	21
Total Unrealized Capital Losses	$ 2	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	15
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,501	97
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 186	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	371
More Than Twelve Months Below Amortized Cost, Fair Value	$ 28	131
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 14	10
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	$ 1,529	228
Total Unrealized Capital Losses	$ 200	13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	380
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,013	75
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 67	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	108
More Than Twelve Months Below Amortized Cost, Fair Value	$ 77	134
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 30	16
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	11
Total, Fair Value	$ 1,090	209
Total Unrealized Capital Losses	$ 97	16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	119
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	$ 686	6
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 68	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	198
More Than Twelve Months Below Amortized Cost, Fair Value	$ 13	53
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 5	3
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	$ 699	59
Total Unrealized Capital Losses	$ 73	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	206
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,557	21
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 131	0
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	144
More Than Twelve Months Below Amortized Cost, Fair Value	$ 36	56
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 7	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	$ 1,593	77
Total Unrealized Capital Losses	$ 138	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	150
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,401	535
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 117	6
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	322
More Than Twelve Months Below Amortized Cost, Fair Value	$ 97	139
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 14	5
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	53
Total, Fair Value	$ 1,498	674
Total Unrealized Capital Losses	$ 131	11
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	375
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 1,295	331
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 231	3
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	291
More Than Twelve Months Below Amortized Cost, Fair Value	$ 0	18
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	$ 1,295	349
Total Unrealized Capital Losses	$ 231	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	291
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Twelve Months or Less below amortized cost, Fair Value	$ 909	217
Twelve Months or Less below amortized cost, Unrealized Capital Loss	$ 99	2
Twelve Months or Less below amortized cost, total number of securities in an unrealized loss position | securities	238
More Than Twelve Months Below Amortized Cost, Fair Value	$ 254	500
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 43	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	$ 1,163	717
Total Unrealized Capital Losses	$ 142	$ 13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | securities	340