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Earnings Per Share - Summary of Earnings Per Share (Parenthetical) (Detail) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Class A Stock [Member]
       
Schedule Of Earnings Per Share Basic And Diluted [Line Items]        
Number of antidilutive warrants, options and restricted shares, for the year 57,573 1,059,638 57,573 1,059,638
CIBC World Markets Corp. [Member] | January 2008 Acquisition [Member]
       
Schedule Of Earnings Per Share Basic And Diluted [Line Items]        
Period for exercise of warrant     5 years  
Acquisition date of CIBC World Markets Corp.     Jan. 14, 2008  
Warrants expire date     2013-04-13  
CIBC World Markets Corp. [Member] | January 2008 Acquisition [Member] | Class A Stock [Member]
       
Schedule Of Earnings Per Share Basic And Diluted [Line Items]        
Company issued a warrant to purchase share of Class A Stock 1,000,000   1,000,000  
Class A Stock exercisable     $ 48.62