Financial Instruments - Contractual Maturities (Details) $ in Millions	Mar. 31, 2020 USD ($)
Amortized Cost
Due within one year	$ 26,936
Due after one year through five years	11,645
Due after five years through ten years	533
Due after ten years	2,108
Cash Equivalents and Marketable Fixed Income Securities, Cost	41,222
Estimated Fair Value
Due within one year	26,935
Due after one year through five years	11,498
Due after five years through ten years	527
Due after ten years	2,100
Cash Equivalents and Marketable Fixed Income Securities, Fair Value	$ 41,060