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Cash Dividend Paid (Detail) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2012
Mar. 31, 2012
Jun. 30, 2012
Jun. 30, 2011
Dividends Payable [Line Items]        
Cash dividend per common share $ 0.16250 $ 0.16125 $ 0.16375  
Total cash dividends paid $ 10,956,000 $ 10,847,000 $ 21,803,000 $ 20,902,000
Record date Apr. 27, 2012 Feb. 24, 2012 Jul. 27, 2012  
Payment date May 18, 2012 Mar. 26, 2012