Cash, Cash Equivalents, and Marketable Securities - Contractual Maturities (Details) $ in Millions	Sep. 30, 2017 USD ($)
Amortized Cost
Due within one year	$ 11,305
Due after one year through five years	4,355
Due after five years through ten years	317
Due after ten years	812
Cash equivalents and marketable securities	16,789
Estimated Fair Value
Due within one year	11,303
Due after one year through five years	4,351
Due after five years through ten years	316
Due after ten years	809
Cash equivalents and marketable securities	$ 16,779