Investments - Summary of Debt Securities with a Decline in Fair Value (Details) $ in Millions	Mar. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($) position
Investment grade
One year or less
Fair Value	$ 4,550	$ 723
Amortized Cost	4,816	729
Unrealized Depreciation	$ (266)	$ (6)
Number of Issues | position	1,023	267
More than one year
Fair Value	$ 194	$ 366
Amortized Cost	226	378
Unrealized Depreciation	$ (32)	$ (12)
Number of Issues | position	34	118
Below investment grade
One year or less
Fair Value	$ 1,539	$ 340
Amortized Cost	1,712	348
Unrealized Depreciation	$ (173)	$ (8)
Number of Issues | position	1,150	355
More than one year
Fair Value	$ 64	$ 84
Amortized Cost	81	88
Unrealized Depreciation	$ (17)	$ (4)
Number of Issues | position	56	93