Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss) available to common shareholders	$ 25,576	$ (9,885)	$ 55,518	$ 20,121
Weighted average common shares outstanding - Basic	38,713	38,248	38,624	38,181
Dilutive effect of stock-based awards	754	0	715	334
Weighted average common shares outstanding - Diluted	39,467	38,248	39,339	38,515
Earnings (loss) per common share - Basic	$ 0.66	$ (0.26)	$ 1.44	$ 0.53
Earnings (loss) per common share - Diluted	$ 0.65	$ (0.26)	$ 1.41	$ 0.52