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Stock Buy Back Plans - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Sep. 30, 2015
Aug. 31, 2015
Jul. 31, 2015
Feb. 28, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Oct. 22, 2014
Equity Class Of Treasury Stock [Line Items]                
Purchase of treasury shares (in shares) 81,229 198,923 114,352 341,020 735,524      
Purchase of treasury shares         $ 28,823,000 $ 18,024,000 $ 13,791,000  
Class A Common Stock [Member]                
Equity Class Of Treasury Stock [Line Items]                
Stock repurchase program, authorized amount               $ 75,000,000
Purchase of treasury shares (in shares)         735,524 501,271    
Class A Common Stock [Member] | Employee Restricted Stock Plan [Member]                
Equity Class Of Treasury Stock [Line Items]                
Stock tendered for payments of withholding taxes (in shares)         77,732 80,772    
Purchase of treasury shares         $ 2,900,000 $ 3,200,000