Cash, Cash Equivalents, and Marketable Securities - Contractual Maturities (Details) $ in Millions	Jun. 30, 2017 USD ($)
Amortized Cost
Due within one year	$ 12,088
Due after one year through five years	3,021
Due after five years through ten years	176
Due after ten years	315
Cash equivalents and marketable securities	15,600
Estimated Fair Value
Due within one year	12,085
Due after one year through five years	3,017
Due after five years through ten years	175
Due after ten years	313
Cash equivalents and marketable securities	$ 15,590