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Performance Units (Detail)
3 Months Ended
Mar. 31, 2020
shares
2019  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 489,800
2018  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 310,700
2017  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 186,198
2016  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 185,000