Cash Equivalents and Investments - Schedule of Available-for-Sale Securities Unrealized Loss Position (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value		$ 39,210	$ 57,309
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss		(85)	(65)
U.S. Government Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value		16,473	24,940
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss		(17)	(29)
Corporate Notes
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value		18,149	24,503
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss		(68)	(36)
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value	[1]	$ 4,588	$ 7,866

[1] Balances less than 12 months include investments that were in an immaterial unrealized loss position as of March 31, 2018 and December 31, 2017.