Employee Stock Purchase Plan, Stock Options and Warrants - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Total intrinsic value of stock options exercised	$ 25,692	$ 9,264	$ 3,648
Cash received from stock option exercises	8,510	5,552	4,577
Excess tax benefit from the exercise of stock options	$ 4,278	$ 2,264	$ 669
Number of Shares
Beginning balance (in shares)	3,623
Granted (in shares)	692
Exercised (in shares)	(690)
Forfeited/expired (in shares)	(118)
Outstanding at December 31 (in shares)	3,507	3,623
Exercisable (in shares)				1,101
Ending vested and expected to vest (in shares)				3,388
Weighted Average Exercise Price
Beginning balance (in dollars per share)	$ 26.30	$ 20.40		$ 26.30
Granted (in dollars per share)	46.45
Exercised (in dollars per share)	15.41
Forfeited/expired (in dollars per share)	26.90
Outstanding at December 31 (in dollars per share)	$ 26.30	$ 20.40
Exercisable (in dollars per share)				17.71
Ending vested and expected to vest (in dollars per share)				$ 26.05
Outstanding, remaining contractual term	4 years 6 months 14 days
Exercisable, remaining contractual term	3 years 3 months 29 days
Ending vested and expected to vest, remaining contractual term	4 years 6 months 3 days
Outstanding, intrinsic value				$ 103,483
Exercisable, intrinsic value				41,963
Ending vested and expected to vest, intrinsic value				$ 100,820