Cash, Cash Equivalents, and Marketable Securities - Contractual Maturities (Details) $ in Millions	Dec. 31, 2018 USD ($)
Amortized Cost
Due within one year	$ 27,520
Due after one year through five years	2,865
Due after five years through ten years	187
Due after ten years	538
Cash Equivalents and Marketable Fixed Income Securities, Cost	31,110
Estimated Fair Value
Due within one year	27,508
Due after one year through five years	2,845
Due after five years through ten years	185
Due after ten years	529
Cash Equivalents and Marketable Fixed Income Securities, Fair Value	$ 31,067