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Compensation Expense Associated with Performance Units (Detail) - Performance Units Awards - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
2017    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense associated with Performance Units $ 482  
2016    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense associated with Performance Units 321 $ 321
2015    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense associated with Performance Units $ 338 338
2014    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense associated with Performance Units   $ 449