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Stock Incentive Plan - Additional Information (Detail)
$ in Millions
3 Months Ended 9 Months Ended
May 09, 2017
USD ($)
Feb. 19, 2017
USD ($)
May 09, 2016
Feb. 15, 2016
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Grant
Sep. 30, 2016
USD ($)
May 31, 2017
Feb. 29, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Grant date fair value fully vested         $ 5.4 $ 2.7 $ 13.6 $ 11.6    
LTI Program                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Performance period     3 years 3 years            
Number of grants expected that achieves the performance conditions | Grant             2      
LTI Program | Non-employee Directors                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Grant date fair value fully vested $ 1.2                  
LTI Program | Annual Base Retainer | Non-employee Directors                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of grants fully-vested shares of common stock             50.00%      
LTI Program | Minimum                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of target number of performance shares to be earned     0.00% 0.00%            
LTI Program | Maximum                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of target number of performance shares to be earned     200.00% 200.00%            
LTI Program | Weighted Average                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of target number of performance shares to be earned     50.00% 50.00%            
LTI Program | Performance Shares                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of awards granted                   50.00%
Grant date fair value   $ 7.9                
LTI Program | Restricted Stock Units (RSUs)                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of awards granted                   50.00%
Grant date fair value   $ 7.9                
LTI Program | Restricted Stock Units (RSUs) | Non-employee Directors                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of awards granted                 50.00%  
Grant date fair value $ 1.0