Quarterly report pursuant to sections 13 or 15(d)

Equity-Based Compensation - Summary of Equity Grants (Detail)

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Equity-Based Compensation - Summary of Equity Grants (Detail) (USD $)
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Deferred Equity Units [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning Balance, Units 329,757 439,067
Granted, Units    19,642
Vested, Units (133,863) (128,953)
Forfeited, Units (4,066)  
Ending Balance, Units 191,828 329,757
Beginning Balance, Fair Value per unit $ 10.47 $ 8.00
Granted, Fair Value per unit    $ 12.29
Vested, Fair Value per unit $ (12.00) $ (12.29)
Forfeited, Fair Value per unit $ (12.00)  
Ending Balance, Fair Value per unit $ 12.00 $ 12.29
Performance Units [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning Balance, Units 257,787 250,716
Granted, Units    9,821
Vested, Units      
Forfeited, Units (13,121) (2,750)
Ending Balance, Units 244,666 257,787
Beginning Balance, Fair Value per unit $ 3.27 $ 3.07
Granted, Fair Value per unit    $ 4.70
Vested, Fair Value per unit      
Forfeited, Fair Value per unit $ (3.75) $ (4.70)
Ending Balance, Fair Value per unit $ 3.75 $ 4.70