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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, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
2017        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 482   $ 803  
2016        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units 321 $ 321 963 $ 642
2015        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units $ 338 338 1,013 1,013
2014        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units   $ 449 $ 449 1,347
2013        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense associated with Performance Units       $ 464