Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Units
Beginning balance (in shares)	14.3	15.9	20.1
Units granted (in shares)	8.0	6.7	5.0
Units vested (in shares)	(5.8)	(6.6)	(7.1)
Units forfeited (in shares)	(1.3)	(1.7)	(2.1)
Ending balance (in shares)	15.2	14.3	15.9
Weighted Average Grant-Date Fair Value
Beginning Balance (in dollars per share)	$ 1,458	$ 1,024	$ 725
Units granted (in dollars per share)	2,373	1,808	1,522
Units vested (in dollars per share)	1,239	827	578
Units forfeited (in dollars per share)	1,642	1,223	862
Ending Balance (in dollars per share)	$ 2,004	$ 1,458	$ 1,024