Stock-Based Compensation - Stock Option Activity (Details) - Stock options $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)
Stock options, Shares Subject to Options
Outstanding at the beginning of the period (in shares) | shares	2,593,310
Exercised (in shares) | shares	(312,783)
Forfeited /Expired (in shares) | shares	(19,165)
Outstanding at the end of the period (in shares) | shares	2,261,362
Exercisable at the end of the period (in shares) | shares	1,520,617
Exercisable and expected to vest at the end of the period (in shares) | shares	2,199,732
Stock options, Weighted Average Option Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 21.41
Exercised (in dollars per share) | $ / shares	18.67
Forfeited (in dollars per share) | $ / shares	21.88
Outstanding at the end of the period (in dollars per share) | $ / shares	21.79
Exercisable at the end of the period (in dollars per share) | $ / shares	20.13
Exercisable and expected to vest at the end of the period (in dollars per share) | $ / shares	$ 21.69
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding	5 years 1 month 6 days
Weighted Average Remaining Contractual Term, Exercisable	4 years 8 months 12 days
Weighted Average Remaining Contractual Term, Exercisable and expected to vest	5 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding | $	$ 63.7
Aggregate Intrinsic Value, Exercisable | $	45.3
Aggregate Intrinsic Value, Exercisable and expected to vest | $	$ 62.2
Fair value of the Company's common stock price (in dollars per share) | $ / shares	$ 49.95
Intrinsic value of options exercised | $	$ 6.9	$ 5.3
Restricted stock award | shares	0
Total fair value of shares vested | $		$ 0.2
Minimum
Stock-Based Compensation
Vesting period	3 years
Maximum
Stock-Based Compensation
Vesting period	4 years