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Dividends Declared Per Share (Detail) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dividends Payable [Line Items]        
Dividend Amount $ 0.345 $ 0.345 $ 1.035 $ 1.035
Group One
        
Dividends Payable [Line Items]        
Declaration Date     Jun. 06, 2012  
Date Paid (Payable)     Aug. 15, 2012  
Shareholder of Record Date     Aug. 01, 2012  
Dividend Amount     $ 0.345  
Group Two
        
Dividends Payable [Line Items]        
Declaration Date     Mar. 22, 2012  
Date Paid (Payable)     May 15, 2012  
Shareholder of Record Date     May 01, 2012  
Dividend Amount     $ 0.345  
Group Three
        
Dividends Payable [Line Items]        
Declaration Date     Jan. 17, 2012  
Date Paid (Payable)     Feb. 15, 2012  
Shareholder of Record Date     Feb. 01, 2012  
Dividend Amount     $ 0.345  
Group Four
        
Dividends Payable [Line Items]        
Declaration Date     Sep. 21, 2011  
Date Paid (Payable)     Nov. 15, 2011  
Shareholder of Record Date     Nov. 01, 2011  
Dividend Amount     $ 0.345  
Group Five
        
Dividends Payable [Line Items]        
Declaration Date     Jun. 16, 2011  
Date Paid (Payable)     Aug. 15, 2011  
Shareholder of Record Date     Aug. 01, 2011  
Dividend Amount     $ 0.345  
Group Six
        
Dividends Payable [Line Items]        
Declaration Date     Mar. 24, 2011  
Date Paid (Payable)     May 16, 2011  
Shareholder of Record Date     May 02, 2011  
Dividend Amount     $ 0.345  
Group Seven
        
Dividends Payable [Line Items]        
Declaration Date     Jan. 18, 2011  
Date Paid (Payable)     Feb. 15, 2011  
Shareholder of Record Date     Feb. 01, 2011  
Dividend Amount     $ 0.345