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Compensation Expense Associated with Performance Units (Detail) - Performance Units Awards - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
2018      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units $ 2,001    
2017      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units 1,927 $ 1,284  
2016      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units 1,285 1,285 $ 963
2015      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units $ 338 1,351 1,351
2014      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units   $ 449 1,796
2013      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense associated with Performance Units     $ 464