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Stock Compensation Plans - Purchased Restricted Stock Plan - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
PRSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Issued Shares (in shares) 66,160 61,991 66,674
Aggregate Purchase Price $ 8.7 $ 8.5 $ 9.3
Total fair value of vested $ 19.5    
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Covered Shares (in shares) 33,080 30,996 33,337
Total fair value of vested $ 4.0    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted $ 10.1    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted, weighted average period, in years 1 year 7 months 6 days