Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Stock options, Shares Subject to Options
Outstanding at the beginning of the period (in shares)	3,235,673
Granted (in shares)	126,260
Exercised (in shares)	(529,357)
Forfeited (in shares)	(186,263)
Outstanding at the end of the period (in shares)	2,646,313
Exercisable at the end of the period (in shares)	1,653,296
Exercisable and expected to vest at the end of the period (in shares)	2,571,939
Stock options, Weighted Average Option Price
Outstanding at the beginning of the period (in dollars per share)	$ 20.16
Granted (in dollars per share)	35.86
Exercised (in dollars per share)	18.03
Forfeited (in dollars per share)	20.19
Outstanding at the end of the period (in dollars per share)	21.33
Exercisable at the end of the period (in dollars per share)	19.35
Exercisable and expected to vest at the end of the period (in dollars per share)	$ 21.23
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding	5 years 9 months 18 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 2 months 12 days
Weighted Average Remaining Contractual Term, Exercisable and expected to vest	5 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding	$ 32.4
Aggregate Intrinsic Value, Exercisable	23.3
Aggregate Intrinsic Value, Exercisable and expected to vest	$ 31.7
Fair value of the Company's common stock price (in dollars per share)	$ 33.45
Weighted average fair values of options granted (in dollars per share)	$ 9.50	$ 7.61
Intrinsic value of options exercised	$ 7.2	$ 10.9