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Reconciliation of Earnings Per Share (Detail) (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
Earnings Per Share Disclosure [Line Items]                      
Net income for basic and diluted earnings per share $ 18,844 $ 18,495 $ 16,952 $ 13,662 $ 17,014 $ 16,276 $ 14,390 $ 10,498 $ 67,953 $ 58,178 $ 43,458
Weighted average shares outstanding - basic                 37,053 36,913 37,223
Dilutive effect of stock options and restricted stock                 132 150 162
Weighted average shares outstanding - diluted                 37,185 37,063 37,385
Earnings per share - basic $ 0.51 $ 0.50 $ 0.46 $ 0.37 $ 0.46 $ 0.44 $ 0.39 $ 0.28 $ 1.83 $ 1.58 $ 1.17
Earnings per share - diluted $ 0.51 $ 0.50 $ 0.46 $ 0.37 $ 0.46 $ 0.44 $ 0.39 $ 0.28 $ 1.83 $ 1.57 $ 1.16