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Earnings Per Common Share (Computation Of Basic And Diluted Net Earnings Per Share) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Earnings Per Common Share [Abstract]                      
Net income $ 10,565,000 $ 9,996,000 $ 9,828,000 $ 7,767,000 $ 9,123,000 $ 9,169,000 $ 8,721,000 $ 7,428,000 $ 38,156,000 $ 34,441,000 $ 30,342,000
Basic earnings per common share, Income (Numerator)                 38,156,000 34,441,000 30,342,000
Basic earnings per common share, Shares (Denominator)                 66,637,000 65,917,000 65,376,000
Basic earnings per common share $ 0.16 [1] $ 0.15 [1] $ 0.15 [1] $ 0.12 [1] $ 0.14 [1] $ 0.14 [1] $ 0.13 [1] $ 0.11 [1] $ 0.57 $ 0.52 $ 0.46
Effect of dilutive securities, Options, Shares                 948,000 1,091,000 1,053,000
Effect of dilutive securities, Options, Per-Share Amount                 $ (0.01) $ (0.01)  
Diluted earnings per common share, Income (Numerator)                 $ 38,156,000 $ 34,441,000 $ 30,342,000
Diluted earnings per common share, Shares (Denominator)                 67,585,000 67,008,000 66,429,000
Diluted earnings per common share, Per-share Amount $ 0.16 [1] $ 0.15 [1] $ 0.15 [1] $ 0.12 [1] $ 0.14 [1] $ 0.14 [1] $ 0.13 [1] $ 0.11 [1] $ 0.56 $ 0.51 $ 0.46
Options outstanding to purchase common stock excluded from computation of diluted earnings per common share                 510,000 1,037,000 528,000
[1] Year-to-date earnings and cash dividends per common share amounts may differ from the sum of quarterly amounts due to rounding.