Cash, Cash Equivalents and Current Marketable Securities - Contractual Maturities of Available for Sale Securities (Details) $ in Millions	Dec. 31, 2017 USD ($)
Cost Basis
Due within one year	$ 5,214
Due after one year through five years	96
Due after five years through ten years	0
Total debt securities	5,310
Fair Value
Due within one year	5,214
Due after one year through five years	96
Due after five years through ten years	0
Total debt securities	$ 5,310