Earnings Per Common Share Computations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Earnings from continuing operations	$ 50,298	$ 48,576	$ 51,357	$ 49,930
Earnings from continuing operations allocated to participating securities	(1,170)	(722)	(1,197)	(743)
Numerator for basic earnings from continuing operations per common share	49,128	47,854	50,160	49,187
Net earnings	50,298	48,576	51,356	49,929
Net earnings allocated to participating securities	(1,170)	(723)	(1,197)	(743)
Numuerator for basic net earnings per common share	49,128	47,853	50,159	49,186
Net Income (Loss) Available to Common Stockholders, Diluted	50,298	48,576	51,357	49,930
Income Loss From Continuing Operations, Diluted	$ 50,298	$ 48,576	$ 51,356	$ 49,929
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in shares)	17,268	16,978	17,239	16,940
Plus dilutive effect of stock options (in shares)	242	268	232	247
Plus dilutive effect of participating securities (in shares)	411	256	411	256
Diluted (in shares)	17,921	17,502	17,882	17,443
Net earnings (in dollars per share), Basic	$ 2.85	$ 2.82	$ 2.91	$ 2.90
Net earnings (in dollars per share), Diluted	$ 2.81	$ 2.77	$ 2.87	$ 2.86