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Schedule of Weighted-Average Restricted Stock Unit Award Fair Value (Detail) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award      
Non-vested at January 1 $ 35,580 $ 29,220 $ 21,710
Awarded 72,760 39,080.00 45,990
Vested 31,530 23,590 20,200
Forfeited 45,360 35,600 24,670
Non-vested at December 31 $ 47,840 $ 35,580 $ 29,220