Fair Value Measurements - Schedule of Available for Sale Securities Maturities (Details) $ in Millions	Sep. 30, 2018 USD ($)
Cost Basis
Due within one year	$ 11,079
Due after one year through five years	77
Due after five years through ten years	0
Total debt securities	11,156
Fair Value
Due within one year	11,078
Due after one year through five years	77
Due after five years through ten years	0
Total debt securities	$ 11,155