Ownership, Description of Business, and Significant Accounting Policies (Details 1) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Components of basic and diluted earnings per share                      
Income from continuing operations $ 20,167 $ 21,308 $ 18,988 $ 18,108 $ 17,235 $ 15,310 $ 14,766 $ 13,282 $ 78,571 $ 60,593 $ 47,394
(Loss) income from discontinued operations, net of tax     (2,392) (6,702) (101) 60 9 397 (9,094) 365 1,658
Net income $ 20,167 $ 21,308 $ 16,596 $ 11,406 $ 17,134 $ 15,370 $ 14,775 $ 13,679 $ 69,477 $ 60,958 $ 49,052
Weighted average common shares outstanding - basic                 24,855 24,572 23,897
Dilutive effect of stock-based awards                 981 1,118 1,194
Weighted average common shares outstanding - diluted                 25,836 25,690 25,091
Basic earnings (loss) per share from:                      
Continuing operations $ 0.82 $ 0.85 $ 0.76 $ 0.73 $ 0.70 $ 0.62 $ 0.60 $ 0.55 $ 3.16 $ 2.47 $ 1.98
Discontinued operations $ 0.00 $ 0.00 $ (0.09) $ (0.27) $ (0.01) $ 0.00 $ 0.00 $ 0.01 $ (0.36) $ 0.01 $ 0.07
Net basic earnings per share                 $ 2.80 $ 2.48 $ 2.05
Diluted earnings (loss) per share from:                      
Continuing operations $ 0.79 $ 0.82 $ 0.74 $ 0.70 $ 0.67 $ 0.60 $ 0.58 $ 0.52 $ 3.04 $ 2.36 $ 1.89
Discontinued operations $ 0.00 $ 0.00 $ (0.10) $ (0.26) $ 0.00 $ 0.00 $ 0.00 $ 0.02 $ (0.35) $ 0.01 $ 0.06
Net diluted earnings per share                 $ 2.69 $ 2.37 $ 1.95