Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) shares
Number of Units
Beginning balance (in shares) | shares	19.8
Units granted (in shares) | shares	7.2
Units vested (in shares) | shares	(4.6)
Units forfeited (in shares) | shares	(1.3)
Ending balance (in shares) | shares	21.1
Weighted-Average Grant-Date Fair Value
Beginning balance | $	$ 506
Units granted | $	918
Units vested | $	387
Units forfeited | $	620
Ending balance | $	$ 666