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Stock-based Compensation - Additional Information (Detail) - shares
1 Months Ended 3 Months Ended 9 Months Ended
Apr. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Number of stock option granted   0 0 0 0
Performance Units Awards          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Performance period       3 years  
Performance Units Awards | 2015          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Shares issued 381,200        
Performance Units Awards | Condition One [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Award description       Generally, the recipients will receive a target number of shares if the Company’s total shareholder return during the performance period, when compared to the peer group, is at the 50th percentile. If the Company’s total shareholder return during the performance period, when compared to the peer group, is at the 75th percentile or higher, then the recipients will receive two times the target number of shares.