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Compensation Expense Associated with Performance Units (Detail) - Performance Units Awards - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
2018        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 667   $ 667  
2017        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units 482 $ 321 963 $ 321
2016        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 321 321 642 642
2015        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units   $ 338 $ 338 675
2014        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units       $ 449