Investments - Estimated Fair Values of Investments Classified as Available for Sale by Date of Contractual Maturity (Details) $ in Millions	Sep. 30, 2016 USD ($)
Investments, Debt and Equity Securities [Abstract]
One year or less	$ 3,317
One year through two years	641
Two years through three years	487
Three years through four years	58
Four years through five years	29
Greater than five years	25
Total	$ 4,557