Quarterly report pursuant to sections 13 or 15(d)

Stockholders' Equity - Additional Information (Detail)

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Stockholders' Equity - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Class of Stock [Line Items]        
Partnership distributions, amount $ 4,231 $ 5,339 $ 17,900 $ 13,194
Amount distributed to existing partners     10,000  
Distributions of partner incentive allocations of net income $ 12,104 $ 6,581 $ 12,104 $ 6,581
Class A Common Stock [Member]
        
Class of Stock [Line Items]        
Common stock shares issued 4,790,694   4,790,694  
Class B Common Stock [Member]
        
Class of Stock [Line Items]        
Common stock shares issued 6,459,316   6,459,316  
IPO [Member] | Class A Common Stock [Member]
        
Class of Stock [Line Items]        
Common stock shares issued 4,790,684   4,790,684  
Employee Stock [Member] | Class B Common Stock [Member]
        
Class of Stock [Line Items]        
Common stock shares issued 6,459,316   6,459,316