Stockholders' Equity, Restricted Stock Unit Activity (Details) - Restricted Stock Units [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares [Abstract]
Non-vested at beginning of period (in shares)	778
Granted (in shares)	420
Vested (in shares)	(296)
Cancelled/forfeited (in shares)	(39)
Non-vested at end of period (in shares)	863	778
Weighted Average Grant Date Fair Value per Share [Abstract]
Non-vested at beginning of period (in dollars per share)	$ 47.68
Granted (in dollars per share)	48.88	$ 41.79	$ 65.69
Vested (in dollars per share)	43.79
Cancelled/forfeited (in dollars per share)	48.90
Non-vested at end of period (in dollars per share)	$ 49.55	$ 47.68
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested RSUs	$ 16.5
Weighted average period for recognition	1 year 2 months 12 days