Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)	(575,372)
Forfeited /Expired (in shares)	(193,251)
Outstanding at the end of the period (in shares)	2,593,310	3,235,673
Exercisable at the end of the period (in shares)	1,789,819
Exercisable and expected to vest at the end of the period (in shares)	2,533,128
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)	17.87
Forfeited (in dollars per share)	20.31
Outstanding at the end of the period (in dollars per share)	21.41	$ 20.16
Exercisable at the end of the period (in dollars per share)	19.77
Exercisable and expected to vest at the end of the period (in dollars per share)	$ 21.33
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding	5 years 7 months 6 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 1 month 6 days
Weighted Average Remaining Contractual Term, Exercisable and expected to vest	5 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding	$ 22.4
Aggregate Intrinsic Value, Exercisable	17.9
Aggregate Intrinsic Value, Exercisable and expected to vest	$ 22.1
Fair value of the Company's common stock price (in dollars per share)	$ 29.77
Weighted average fair values of options granted (in dollars per share)	$ 9.50	$ 7.61	$ 7.72
Intrinsic value of options exercised	$ 8.0	$ 16.2	$ 11.2
Additional share-based compensation disclosures
Unrecognized pre-tax stock-based compensation expense	$ 4.4
Weighted average remaining service period	2 years 29 days