Stock-Based Compensation (RSU Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Beginning Balance	1,094,161
Granted	265,105
Additional shares vested	94,036
Released from restrictions	(682,850)
Forfeited	(48,528)
Ending Balance	721,924
Vested or expected vest, at end of period	687,385
Weighted Average Fair Value
Beginning Balance (in dollars per share)	$ 119.26
Granted (in dollars per share)	91.84
Additional shares vested (in dollars per share)	89.67
Released from restrictions (in dollars per share)	70.49
Forfeited (in dollars per share)	86.52
Ending Balance (in dollars per share)	95.65
Vested or expected to vest, at end of period (in dollars per share)	$ 95.71
Intrinsic Value (in millions)
Outstanding	$ 69	$ 108
Vested or expected to vest, at end of period	$ 66