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Summary of Nonvested Restricted Shares Activity (Detail) - Restricted Stock Units (RSUs)
12 Months Ended
Dec. 31, 2020
$ / shares
shares
GRANT DATE FAIR VALUE  
Nonvested awards outstanding, beginning balance | $ / shares $ 65.92 [1]
Granted | $ / shares 76.78 [1]
Vested | $ / shares 69.46 [1]
Forfeited | $ / shares 71.47 [1]
Nonvested awards outstanding, ending balance | $ / shares $ 70.49 [1]
NONVESTED NUMBER OF SHARES  
Nonvested awards outstanding, beginning balance | shares 150,500
Granted | shares 94,400
Vested | shares (96,700)
Forfeited | shares (5,800)
Nonvested awards outstanding, ending balance | shares 142,400
[1] Weighted Average