Earnings Per Common Share Computations (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Earnings from continuing operations	$ 48,576	$ 39,990	$ 49,930	$ 36,804
Earnings from continuing operations allocated to participating securities	(722)	(906)	(743)	(869)
Numerator for basic earnings from continuing operations per common share	47,854	39,084	49,187	35,935
Net earnings	48,576	40,147	49,929	36,962
Net earnings allocated to participating securities	(723)	(910)	(743)	(873)
Numuerator for basic net earnings per common share	47,853	39,237	49,186	36,089
Discontinued operations, net of income taxes	0	157	(1)	158
Net earnings	$ 48,576	$ 40,147	$ 49,929	$ 36,962
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in shares)	16,978,000	16,444,000	16,940,000	16,401,000
Plus dilutive effect of stock options (in shares)	268,000	109,000	247,000	100,000
Plus dilutive effect of participating securities (in shares)	256,000	382,000	256,000	398,000
Diluted (in shares)	17,502,000	16,935,000	17,443,000	16,899,000
Earnings Per Share, Basic [Abstract]
Earnings from continuing operations (per share, basic)	$ 2.82	$ 2.38	$ 2.90	$ 2.19
Discontinued operations (per share, basic)	$ 0.00	$ 0.01	$ 0.00	$ 0.01
Net earnings (per basic share)	$ 2.82	$ 2.39	$ 2.90	$ 2.20
Earnings Per Share, Diluted [Abstract]
Earnings from continuing operations (per share, diluted)	$ 2.77	$ 2.36	$ 2.86	$ 2.18
Discontinued operations (per share, diluted)	$ 0.00	$ 0.01	$ 0.00	$ 0.01
Net earnings (per share, diluted)	$ 2.77	$ 2.37	$ 2.86	$ 2.19
Number Of Shares Excluded From Computation Of Diluated Loss Per Common Share (in shares)		18,000		18,000