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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income (loss) available to common shareholders	$ 19,882	$ 25,576	$ (123,127)	$ 55,518
Weighted average common shares outstanding - Basic (in shares)	39,340	38,713	39,177	38,624
Dilutive effect of stock-based awards (in shares)	569	754	0	715
Weighted average common shares outstanding - Diluted (in shares)	39,909	39,467	39,177	39,339
Earnings (loss) per common share - Basic (in dollars per share)	$ 0.51	$ 0.66	$ (3.14)	$ 1.44
Earnings (loss) per common share - Diluted (in dollars per share)	$ 0.50	$ 0.65	$ (3.14)	$ 1.41