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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Total intrinsic value of stock options exercised	$ 9,264	$ 3,648	$ 7,548
Cash received from stock option exercises	5,552	4,577	6,227
Excess tax benefit from the exercise of stock options	$ 2,264	$ 669	$ 2,124
Number of Shares
Beginning balance (in shares)	2,817
Granted (in shares)	1,297
Exercised (in shares)	(404)
Forfeited/expired (in shares)	(87)
Outstanding at December 31 (in shares)	3,623	2,817
Exercisable (in shares)				1,110
Ending vested and expected to vest (in shares)				3,484
Weighted Average Exercise Price
Beginning balance (in dollars per share)	$ 20.40	$ 15.32		$ 20.40
Granted (in dollars per share)	29.31
Exercised (in dollars per share)	14.02
Forfeited/expired (in dollars per share)	18.79
Outstanding at December 31 (in dollars per share)	$ 20.40	$ 15.32
Exercisable (in dollars per share)				14.35
Ending vested and expected to vest (in dollars per share)				$ 20.23
Outstanding, remaining contractual term	4 years 6 months 26 days
Exercisable, remaining contractual term	2 years 6 months 18 days
Ending vested and expected to vest, remaining contractual term	4 years 6 months 7 days
Outstanding, intrinsic value				$ 82,615
Exercisable, intrinsic value				32,019
Ending vested and expected to vest, intrinsic value				$ 80,052