Earnings Per Share - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Income Attributable to Parent	$ 32,773	$ 45,982	$ 14,133	$ (23,790)	$ 16,173	$ 37,190	$ 14,199	$ (10,440)	$ 69,098	$ 57,122	$ 60,485
Weighted average common stock outstanding, basic (in shares)									39,795	39,557	39,337
Dilutive effect of stock options and restricted stock units									577	668	531
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements									577	668	531
Weighted average common stock outstanding, diluted (in shares)									40,372	40,225	39,868
Basic (in dollars per share)	$ 0.82	$ 1.15	$ 0.35	$ (0.60)	$ 0.41	$ 0.94	$ 0.36	$ (0.27)	$ 1.74	$ 1.44	$ 1.54
Diluted (in dollars per share)	$ 0.81	$ 1.14	$ 0.35	$ (0.60)	$ 0.40	$ 0.92	$ 0.35	$ (0.27)	$ 1.71	$ 1.42	$ 1.52