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Computation of Basic and Diluted Income Per Common 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 [Line Items]                      
Net income $ 61,031 $ 39,791 $ 45,154 $ 17,844 $ 39,465 $ 41,777 $ 39,368 $ 37,417 $ 163,820 $ 158,027 $ 205,435
Basic common shares outstanding                 96,384 99,281 101,678
Stock options and unvested restricted stock                 1,113 1,095 930
Diluted common shares outstanding                 97,497 100,376 102,608
Basic income per common share $ 0.63 $ 0.41 $ 0.47 $ 0.18 $ 0.41 $ 0.42 $ 0.39 $ 0.37 $ 1.70 $ 1.59 $ 2.02
Diluted income per common share $ 0.63 $ 0.41 $ 0.46 $ 0.18 $ 0.40 $ 0.42 $ 0.39 $ 0.37 $ 1.68 $ 1.57 $ 2.00