Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) shares
Number of Units
Beginning balance (in shares) | shares	19.8
Units granted (in shares) | shares	6.1
Units vested (in shares) | shares	(3.5)
Units forfeited (in shares) | shares	(0.9)
Ending balance (in shares) | shares	21.5
Weighted-Average Grant-Date Fair Value
Beginning balance | $	$ 506
Units granted | $	906
Units vested | $	382
Units forfeited | $	592
Ending balance | $	$ 636