Financial Instruments - Contractual Maturities (Details) $ in Millions	Mar. 31, 2019 USD ($)
Amortized Cost
Due within one year	$ 23,482
Due after one year through five years	4,274
Due after five years through ten years	190
Due after ten years	578
Cash Equivalents and Marketable Fixed Income Securities, Cost	28,524
Estimated Fair Value
Due within one year	23,478
Due after one year through five years	4,277
Due after five years through ten years	190
Due after ten years	571
Cash Equivalents and Marketable Fixed Income Securities, Fair Value	$ 28,516