Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net Income	$ 29,718	$ 38,935	$ 28,740	$ 37,350	$ (9,832)	$ 20,569	$ 20,570	$ 18,937	$ 134,743	$ 50,244	$ 64,080
Denominator:
Weighted average shares (in shares)									31,570	31,419	30,276
Dilutive potential common shares (in shares)									597	884	810
Weighted average fully diluted shares (in shares)									32,167	32,303	31,086
Earnings per share:
Basic (in dollars per share)	$ 0.94	$ 1.22	$ 0.91	$ 1.19	$ (0.31)	$ 0.65	$ 0.65	$ 0.60	$ 4.27	$ 1.60	$ 2.12
Diluted (in dollars per share)	$ 0.93	$ 1.20	$ 0.89	$ 1.16	$ (0.31)	$ 0.64	$ 0.64	$ 0.59	$ 4.19	$ 1.56	$ 2.06
Outstanding common stock equivalents having no dilutive effect (in shares)									18	2	18