Investments - Estimated Fair Values of Investments Classified as Available for Sale by Date of Contractual Maturity (Details) $ in Millions	Sep. 30, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
One year or less	$ 1,119
One year through two years	513
Two years through three years	116
Three years through four years	115
Four years through five years	0
Greater than five years	8
Total	$ 1,871