Net Income per Share - Shares and EPS (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
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income									$ 126,633	$ 92,617	$ 89,734
Basic weighted-average shares outstanding									46,213	47,486	48,084
Dilutive effect of potential common stock equivalents - stock options (in shares)									327	288	211
Diluted weighted-average shares outstanding									46,540	47,774	48,295
Net earnings per share:
Basic	$ 0.28	$ 0.96	$ 0.95	$ 0.55	$ 0.28	$ 0.60	$ 0.59	$ 0.49	$ 2.74	$ 1.95	$ 1.87
Diluted	$ 0.28	$ 0.95	$ 0.94	$ 0.54	$ 0.27	$ 0.59	$ 0.59	$ 0.48	$ 2.72	$ 1.94	$ 1.86