Earnings per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares
Earnings per share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	590,000		490,000
Calculation of basic and diluted earnings per share
Net income (loss) | $	$ (57,508)	$ 59,780	$ (110,566)	$ 125,387
Basic weighted-average common shares outstanding	67,483,000	67,069,000	67,473,000	67,063,000
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	0	1,170,000	0	1,117,000
Diluted weighted-average common shares outstanding	67,483,000	68,239,000	67,473,000	68,180,000
Basic net income (loss) per common share | $ / shares	$ (0.85)	$ 0.89	$ (1.64)	$ 1.87
Diluted net income (loss) per common share | $ / shares	$ (0.85)	$ 0.88	$ (1.64)	$ 1.84
Performance Shares [Member]
Earnings per share
Share-based Compensation, Awards Other Than Options, Performance Measurement Period	3		3