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Stock-Based Incentive Plans - Additional Information (Detail)
$ in Millions
6 Months Ended
Jun. 30, 2015
USD ($)
Plans
shares
Jun. 30, 2015
USD ($)
Plans
shares
Dec. 31, 2014
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of benefit plans | Plans 5 5  
Issuance of restricted class A common stock, net   179,799  
Restricted Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Issuance of restricted class A common stock, net   190,674  
Restricted shares, vesting period   3 years  
Number of shares of restricted class A common stock outstanding 869,948 869,948 919,719
Unrecognized compensation cost relating to nonvested share-based compensation | $ $ 24.2 $ 24.2  
Unrecognized compensation cost expected to be recognized over weighted average period 1 year 6 months    
Vest in 2015 [Member] | Restricted Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares of restricted class A common stock outstanding 227,761 227,761  
Vest in 2016 [Member] | Restricted Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares of restricted class A common stock outstanding 416,339 416,339  
Vest in 2017 [Member] | Restricted Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares of restricted class A common stock outstanding 225,598 225,598  
2013 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares available under plan 748,855 748,855  
2013 Manager Plan [Member] | Maximum [Member] | Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares available under plan 2,160,106 2,160,106  
Expired Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares available under plan 0 0