Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Units
Beginning balance (in shares)	19.8	18.9	17.4
Units granted (in shares)	8.9	9.3	9.8
Units vested (in shares)	(6.8)	(6.1)	(5.6)
Units forfeited (in shares)	(1.8)	(2.3)	(2.7)
Ending balance (in shares)	20.1	19.8	18.9
Weighted Average Grant-Date Fair Value
Beginning balance	$ 506	$ 362	$ 285
Units granted	946	660	426
Units vested	400	321	253
Units forfeited	649	440	321
Ending balance	$ 725	$ 506	$ 362