Investments - Summary of Debt Securities with a Decline in Fair Value (Details) $ in Millions	Sep. 30, 2020 USD ($) position	Dec. 31, 2019 USD ($) position
More than one year
Total Fair Value	$ 2,089	$ 1,513
Total Amortized Cost	2,196	1,543
Total Unrealized Depreciation	$ (107)	$ (30)
Total Number of Issues | position	947	833
Investment grade
One year or less
Fair Value	$ 1,193	$ 723
Amortized Cost	1,224	729
Unrealized Depreciation	$ (31)	$ (6)
Number of Issues | position	356	267
More than one year
Fair Value	$ 107	$ 366
Amortized Cost	111	378
Unrealized Depreciation	$ (4)	$ (12)
Number of Issues | position	19	118
Below investment grade
One year or less
Fair Value	$ 713	$ 340
Amortized Cost	774	348
Unrealized Depreciation	$ (61)	$ (8)
Number of Issues | position	536	355
More than one year
Fair Value	$ 76	$ 84
Amortized Cost	87	88
Unrealized Depreciation	$ (11)	$ (4)
Number of Issues | position	36	93