Investments - Contractual Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 453
Due after one year through five years	957
Due after five years through ten years	171
Due after ten years	360
Amortized Cost	1,941	$ 1,692
Estimated Fair Value
Due in one year or less	453
Due after one year through five years	962
Due after five years through ten years	171
Due after ten years	360
Estimated Fair Value	$ 1,946