Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Stock options, Shares Subject to Options
Outstanding at the beginning of the period (in shares)	2,593,310
Exercised (in shares)	(167,177)
Forfeited /Expired (in shares)	(6,212)
Outstanding at the end of the period (in shares)	2,419,921
Exercisable at the end of the period (in shares)	1,658,792
Exercisable and expected to vest at the end of the period (in shares)	2,356,595
Stock options, Weighted Average Option Price
Outstanding at the beginning of the period (in dollars per share)	$ 21.41
Exercised (in dollars per share)	19.02
Forfeited (in dollars per share)	21.57
Outstanding at the end of the period (in dollars per share)	21.58
Exercisable at the end of the period (in dollars per share)	19.94
Exercisable and expected to vest at the end of the period (in dollars per share)	$ 21.48
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding	5 years 3 months 18 days
Weighted Average Remaining Contractual Term, Exercisable	4 years 10 months 24 days
Weighted Average Remaining Contractual Term, Exercisable and expected to vest	5 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding	$ 40.8
Aggregate Intrinsic Value, Exercisable	30.7
Aggregate Intrinsic Value, Exercisable and expected to vest	$ 40.0
Fair value of the Company's common stock price (in dollars per share)	$ 38.44
Intrinsic value of options exercised	$ 3.1	$ 2.8
Restricted stock	0		0
Total fair value of shares vested		$ 0.1
Minimum
Stock-Based Compensation
Vesting period	3 years
Maximum
Stock-Based Compensation
Vesting period	4 years