Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Computation of basic and diluted net income per common share
Net income	$ 20,230	$ 26,418	$ 27,847	$ 31,983
Weighted average common shares outstanding during the period (for basic calculation)	39,494	39,420	39,496	39,399
Dilutive effect of other potential common shares	106	43	108	59
Weighted average common shares and potential common shares (for diluted calculation)	39,600	39,463	39,604	39,458
Net income per common share - Basic	$ 0.51	$ 0.67	$ 0.71	$ 0.81
Net income per common share - Diluted	$ 0.51	$ 0.67	$ 0.70	$ 0.81
Stock awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive stock awards	0	300	100	200