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Equity-Based Compensation - Restricted Stock Awards with Market Conditions Additional Information (Detail) - Restricted stock
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 28, 2014
trading_day
$ / shares
shares
Sep. 30, 2017
USD ($)
shares
Jun. 30, 2017
shares
Mar. 31, 2017
shares
Sep. 30, 2016
USD ($)
Jun. 30, 2016
shares
Mar. 31, 2016
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Dec. 31, 2011
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Number of shares granted (in shares)     54,310 286,256   112,397 434,283      
Number of shares, forfeited (in shares)       167,712   7,500 588,819      
2014 Management Market-Based Awards                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Number of shares granted (in shares) 1,277,500                 366,666
Share-based payment award, threshold, number of trading days | trading_day 45                  
Vesting percentage 25.00%                  
Number of shares, forfeited (in shares)   20,000                
2014 Management Market-Based Awards | Tranche one                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Volume weighted average price targets (in dollars per share) | $ / shares $ 28.07                  
2014 Management Market-Based Awards | Tranche two                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Volume weighted average price targets (in dollars per share) | $ / shares 30.16                  
2014 Management Market-Based Awards | Tranche three                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Volume weighted average price targets (in dollars per share) | $ / shares 32.38                  
2014 Management Market-Based Awards | Tranche four                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Volume weighted average price targets (in dollars per share) | $ / shares $ 34.71                  
2014 Employee Market-Based Awards                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Number of shares granted (in shares)   90,800                
Share-based payment award, threshold, number of trading days | trading_day 45                  
Number of shares, forfeited (in shares)   12,600                
2014 Management and Employee Market Based Awards                    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                    
Share based compensation expense (less than) | $   $ 0.1     $ 0.1     $ 0.1 $ 0.8