Earnings Per Common Share Computations (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
Earnings Per Share [Abstract]
Net earnings	$ 55,044	$ 57,333	$ 53,486	$ 56,633
Net earnings allocated to participating securities	(913)	(674)	(889)	(668)
Numerator for basic earnings per common share	(54,131)	(56,659)	(52,597)	(55,965)
Numerator for diluted earnings per common share	$ 55,044	$ 57,333	$ 53,486	$ 56,633
Earnings Per Share, Basic and Diluted [Abstract]
Basic	17,331	17,531	17,300	17,475
Plus dilutive effect of stock options and restricted stock	112	140	106	200
Plus dilutive effect of participating securities	292	209	292	209
Diluted	17,735	17,880	17,698	17,884
Earnings Per Share, Diluted [Abstract]
Basic	$ 3.12	$ 3.23	$ 3.04	$ 3.20
Diluted	$ 3.10	$ 3.21	$ 3.02	$ 3.17