Stock-based Compensation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding at beginning of period	5,109
Granted	330
Exercised	(2,101)
Forfeited	(126)
Outstanding at end of period	3,212	5,109
Exercisable at end of period	2,527
Vested and expected to vest at end of period	3,178
Weighted-Average Exercise Price
Outstanding at beginning of period	$ 28.88
Granted	36.72
Exercised	28.76
Forfeited	31.09
Outstanding at end of period	29.66	$ 28.88
Exercisable at end of period	28.95
Vested and expected to vest at end of period	$ 29.63
Weighted-Average Remaining Contractual Term (in years)
Outstanding at beginning of period	5 years 5 months	5 years 7 months
Exercisable at end of period	4 years 7 months
Vested and expected to vest at end of period	5 years 5 months
Outstanding at end of period	5 years 5 months	5 years 7 months
Aggregate Intrinsic Value
Outstanding at end of period	$ 54,493
Exercisable at end of period	44,663
Vested and expected to vest at end of period	$ 54,001