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Stock-based Compensation Plans - Additional Information 3 (Detail) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Performance-based PG-PSUs and RG-PSUs    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Vesting period 3 years  
Stock-based compensation expense $ 1.7 $ 5.6
Performance Based TSR - PSUs    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Vesting period 3 years  
Stock-based compensation expense $ 1.3 $ 5.7