Cash, Cash Equivalents, and Marketable Securities - Contractual Maturities (Details) $ in Millions	Dec. 31, 2017 USD ($)
Amortized Cost
Due within one year	$ 17,293
Due after one year through five years	4,149
Due after five years through ten years	302
Due after ten years	575
Cash equivalents and marketable securities	22,319
Estimated Fair Value
Due within one year	17,287
Due after one year through five years	4,129
Due after five years through ten years	300
Due after ten years	570
Cash equivalents and marketable securities	$ 22,286