Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Details) $ in Millions	Sep. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	$ 2,816	$ 160
Gross Unrealized Losses Less than 12 months	(61)	(3)
Fair Value, 12 Months or longer	36	84
Gross Unrealized Losses, 12 months or longer	(1)	(3)
Total Fair Value	2,852	244
Total Gross Unrealized Losses	$ (62)	$ (6)
Total number of available-for-sale securities in an unrealized loss position less than twelve months | Security	304	19
Total number of available-for-sale securities in an unrealized loss position twelve months or longer | Security	6	10
Total number of available-for-sale securities in an unrealized loss position | Security	310	29
Asset-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	$ 426
Gross Unrealized Losses Less than 12 months	(8)
Fair Value, 12 Months or longer	0
Gross Unrealized Losses, 12 months or longer	0
Total Fair Value	426
Total Gross Unrealized Losses	(8)
Commercial mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	166
Gross Unrealized Losses Less than 12 months	(4)
Fair Value, 12 Months or longer	0
Gross Unrealized Losses, 12 months or longer	0
Total Fair Value	166
Total Gross Unrealized Losses	(4)
Corporates
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	1,809	$ 98
Gross Unrealized Losses Less than 12 months	(44)	(2)
Fair Value, 12 Months or longer	36	51
Gross Unrealized Losses, 12 months or longer	(1)	(1)
Total Fair Value	1,845	149
Total Gross Unrealized Losses	(45)	(3)
Hybrids
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	205
Gross Unrealized Losses Less than 12 months	(3)
Fair Value, 12 Months or longer	0
Gross Unrealized Losses, 12 months or longer	0
Total Fair Value	205
Total Gross Unrealized Losses	(3)
Municipals
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	133
Gross Unrealized Losses Less than 12 months	(1)
Fair Value, 12 Months or longer	0
Gross Unrealized Losses, 12 months or longer	0
Total Fair Value	133
Total Gross Unrealized Losses	(1)
Residential mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	17
Gross Unrealized Losses Less than 12 months	(1)
Fair Value, 12 Months or longer	0
Gross Unrealized Losses, 12 months or longer	0
Total Fair Value	17
Total Gross Unrealized Losses	(1)
U.S. Government
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	49	62
Gross Unrealized Losses Less than 12 months	0	(1)
Fair Value, 12 Months or longer	0	0
Gross Unrealized Losses, 12 months or longer	0	0
Total Fair Value	49	62
Total Gross Unrealized Losses	0	(1)
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair Values, Less than 12 months	11	0
Gross Unrealized Losses Less than 12 months	0	0
Fair Value, 12 Months or longer	0	33
Gross Unrealized Losses, 12 months or longer	0	(2)
Total Fair Value	11	33
Total Gross Unrealized Losses	$ 0	$ (2)