Net Income Per Common 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	$ 13,848	$ 31,661	$ 92,723	$ 63,835
Computation of basic and diluted net income per common share
Weighted average common shares outstanding during the period (for basic calculation)	38,998	38,660	38,920	38,601
Dilutive effect of other potential common shares	463	479	477	361
Weighted average common shares and potential common shares (for diluted calculation)	39,461	39,139	39,397	38,962
Net income per common share - Basic	$ 0.36	$ 0.82	$ 2.38	$ 1.65
Net income per common share - Diluted	$ 0.35	$ 0.81	$ 2.35	$ 1.64
Stock awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive stock awards	0	0	100	200

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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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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