Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) shares
Number of Units
Beginning balance (in shares) | shares	20.1
Units granted (in shares) | shares	0.7
Units vested (in shares) | shares	(1.1)
Units forfeited (in shares) | shares	(0.6)
Ending balance (in shares) | shares	19.1
Weighted-Average Grant-Date Fair Value
Beginning balance | $	$ 725
Units granted | $	1,464
Units vested | $	440
Units forfeited | $	750
Ending balance | $	$ 769