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Net Income Per Share - Reconciliation Between Basic and Diluted Weighted Average Shares Outstanding (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Net Income Per Share [Abstract]        
Net income $ 49,504 $ 44,173 $ 141,408 $ 127,501
Net income attributable to unvested awarded performance stock (1,428) (1,395) (4,174) (3,959)
Net income attributable to common shares $ 48,076 $ 42,778 $ 137,234 $ 123,542
Weighted average number of common shares outstanding - basic 143,607 143,212 143,418 142,913
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (4,142) (4,522) (4,233) (4,438)
Weighted average number of common shares outstanding for basic earnings per common share 139,465 138,690 139,185 138,475
Dilutive effect of stock options 2,632 1,753 2,584 1,645
Weighted average number of shares outstanding - diluted 142,097 140,443 141,769 140,120
Basic $ 0.34 $ 0.31 $ 0.99 $ 0.89
Diluted $ 0.34 $ 0.30 $ 0.97 $ 0.88