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Share-Based Compensation - Narrative (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Mar. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation   $ 2.2 $ 2.1 $ 6.4 $ 6.4
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        
Dividend payable $ 0.0        
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       813,361  
Dividend payable   $ 0.0   $ 0.0  
Grant date share price   $ 7.31   $ 7.31  
Payment description       payable in three equal installments over a period of three years after the grant date  
Payment term       3 years