Investments - Schedule of maturities for short-term investments (Details) $ in Millions	Sep. 30, 2017 USD ($)
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Due in one year or less	$ 412
Due after one year through three years	491
Due after three years through five years	37
Due after five years	20
Total	$ 960