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Compensation Expense Associated with Performance Units (Detail) - Performance Units Awards - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
2019        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 830   $ 1,660  
2018        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units 667 $ 667 2,001 $ 1,334
2017        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 482 482 1,445 1,445
2016        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units   $ 321 $ 321 963
2015        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units       $ 338