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Restricted Stock Granted (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock grants 296,017    
Restricted stock grants, Weighted average grant date fair value $ 32.62    
Restricted Stock
     
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock grants 296,017 258,284 240,094
Restricted stock grants, Weighted average grant date fair value $ 32.62 $ 35.27 $ 27.36
Restricted Stock | Minimum
     
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock vesting period, maximum (years) 3 years 3 years 3 years
Restricted Stock | Maximum
     
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock vesting period, maximum (years) 5 years 5 years 5 years
Restricted Stock | Employees
     
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock grants 276,017 246,284 228,094
Restricted Stock | Outside Directors
     
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted stock grants 20,000 12,000 12,000