Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income (loss) available to common shareholders	$ 2,657	$ 14,097	$ (143,009)	$ 29,942
Weighted average common shares outstanding - Basic (in shares)	39,243	38,683	39,095	38,579
Dilutive effect of stock-based awards (in shares)	546	649	0	695
Weighted average common shares outstanding - Diluted (in shares)	39,789	39,332	39,095	39,274
Earnings (loss) per common share - Basic (in dollars per share)	$ 0.07	$ 0.36	$ (3.66)	$ 0.78
Earnings (loss) per common share - Diluted (in dollars per share)	$ 0.07	$ 0.36	$ (3.66)	$ 0.76

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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