Cash, Cash Equivalents, and Marketable Securities - Contractual Maturities (Details) $ in Millions	Jun. 30, 2018 USD ($)
Amortized Cost
Due within one year	$ 16,023
Due after one year through five years	2,895
Due after five years through ten years	216
Due after ten years	433
Cash equivalents and marketable securities	19,567
Estimated Fair Value
Due within one year	16,016
Due after one year through five years	2,864
Due after five years through ten years	212
Due after ten years	424
Cash equivalents and marketable securities	$ 19,516