Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Computation of basic and diluted net income per common share
Net income	$ 19,228	$ 20,230	$ 24,178	$ 27,847
Weighted average common shares outstanding during the period (for basic calculation)	38,814	39,494	38,834	39,496
Dilutive effect of other potential common shares	158	106	16	108
Weighted average common shares and potential common shares (for diluted calculation)	38,972	39,600	38,850	39,604
Net income per common share - Basic	$ 0.50	$ 0.51	$ 0.62	$ 0.71
Net income per common share - Diluted	$ 0.49	$ 0.51	$ 0.62	$ 0.70
Stock awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive stock awards	200	0	600	100

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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