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Stock Buy Back Plans - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Nov. 26, 2012
Class A Common Stock
Dec. 31, 2012
Class A Common Stock
Dec. 31, 2012
Class A Common Stock
Employee Restricted Stock Plan
Dec. 31, 2011
Class A Common Stock
Employee Restricted Stock Plan
Class of Stock [Line Items]                
Stock repurchase program, authorized amount         $ 25,000,000      
Purchase of treasury shares (in shares) 347,592         347,592 55,463 43,247
Purchase of treasury shares   $ 13,020,000 $ 1,523,000 $ 25,070,000     $ 1,800,000 $ 1,500,000