Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) shares
Number of Units
Beginning balance (in shares) | shares	20.1
Units granted (in shares) | shares	3.7
Units vested (in shares) | shares	(3.6)
Units forfeited (in shares) | shares	(1.2)
Ending balance (in shares) | shares	19.0
Weighted-Average Grant-Date Fair Value
Beginning balance | $	$ 725
Units granted | $	1,437
Units vested | $	552
Units forfeited | $	808
Ending balance | $	$ 891