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 $)
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Deferred Equity Units
   
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    $ 12.84
Vested, Fair Value per unit $ (12.00) $ (12.84)
Forfeited, Fair Value per unit $ (12.00)  
Ending Balance, Fair Value per unit   $ 12.84
Deferred Equity Units | Minimum
   
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Ending Balance, Fair Value per unit $ 12.00  
Deferred Equity Units | Maximum
   
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Ending Balance, Fair Value per unit $ 13.63  
Performance Units
   
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Beginning Balance, Units 257,787 250,716
Granted, Units    9,821
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
Forfeited, Fair Value per unit $ (3.75) $ (4.70)
Ending Balance, Fair Value per unit $ 3.75 $ 4.70