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Share-Based Compensation - Narrative (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Share-based compensation $ 1.2 $ 1.9
Restricted stock units    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Annual share-based compensation grants 683,076  
Grant date fair value $ 6.10  
Graded vesting period 3 years  
Performance share units    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Annual share-based compensation grants 683,076  
Grant date fair value $ 6.10  
Cliff vesting period 34 months  
Phantom stock units    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Annual share-based compensation grants 798,105  
Grant date share price $ 7.31  
Payment description payable in three equal installments over a period of three years after the grant date  
Payment term 3 years  
Dividend payable $ 0.0