Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Stock options, Shares Subject to Options
Outstanding at the beginning of the period (in shares)	3,235,673
Exercised (in shares)	(402,354)
Forfeited (in shares)	(161,971)
Outstanding at the end of the period (in shares)	2,671,348
Exercisable at the end of the period (in shares)	1,441,435
Exercisable and expected to vest at the end of the period (in shares)	2,526,346
Stock options, Weighted Average Option Price
Outstanding at the beginning of the period (in dollars per share)	$ 20.16
Exercised (in dollars per share)	17.65
Forfeited (in dollars per share)	20.11
Outstanding at the end of the period (in dollars per share)	20.53
Exercisable at the end of the period (in dollars per share)	18.92
Exercisable and expected to vest at the end of the period (in dollars per share)	$ 20.42
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding	6 years 2 months 12 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 6 months
Weighted Average Remaining Contractual Term, Exercisable and expected to vest	6 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 22.8
Aggregate Intrinsic Value, Exercisable	14.6
Aggregate Intrinsic Value, Exercisable and expected to vest	$ 21.8
Fair value of the Company's common stock price (in dollars per share)	$ 29.04
Weighted average fair values of options granted (in dollars per share)	$ 0.00	$ 7.57
Intrinsic value of options exercised	$ 5.3	$ 6.7