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Subsequent Event - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended 1 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Oct. 31, 2012
Subsequent Event
Subsequent Event [Line Items]          
Special dividend per share of Common and Class B common stock         $ 5.00
Quarterly cash dividend per share of Common and Class B common stock paid $ 0.62 $ 0.57 $ 1.86 $ 1.66 $ 0.62
Dividends payable, date declared, month and year     Oct. 01, 2012    
Dividends payable, date of record     Oct. 15, 2012    
Dividend payment, date, day month and year     Oct. 31, 2012