Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Computation of basic and diluted net income per common share
Net income	$ 22,154	$ 19,228	$ 32,174	$ 24,178
Weighted average common shares outstanding during the period (for basic calculation)	38,643	38,814	38,572	38,834
Dilutive effect of other potential common shares	359	158	359	16
Weighted average common shares and potential common shares (for diluted calculation)	39,002	38,972	38,931	38,850
Net income per common share - Basic	$ 0.57	$ 0.50	$ 0.83	$ 0.62
Net income per common share - Diluted	$ 0.57	$ 0.49	$ 0.83	$ 0.62
Stock awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive stock awards	0	200	100	600

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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