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Share-Based Compensation - Stock Option Activity under Company's Plan - Long Term Incentive Plan (Detail) - Long Term Incentive Plan Stock Option [Member] - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding at January 1, 2021, Number of Shares 50,950  
Granted, Number of Shares 0 25,500
Exercised, Number of Shares (7,000) 0
Forfeited, Number of Shares 0  
Outstanding (vested and expected to vest) at June 30, 2021, Number of Shares 43,950  
Exercisable at June 30, 2021, Number of Shares 27,450  
Outstanding at January 1, 2021, Weighted Average Exercise Price per Share $ 54.81  
Exercised, Weighted Average Exercise Price per Share 47.66  
Outstanding (vested and expected to vest) at June 30, 2021, Weighted Average Exercise Price per Share 55.95  
Exercisable at June 30, 2021, Weighted Average Exercise Price per Share $ 58.91  
Outstanding (vested and expected to vest) at June 30, 2021, Weighted Average Remaining Contractual Term 6 years 8 months 12 days  
Exercisable at June 30, 2021, Weighted Average Remaining Contractual Term 5 years 6 months  
Outstanding (vested and expected to vest) at June 30, 2021, Aggregate Intrinsic Value $ 802  
Exercisable at June 30, 2021, Aggregate Intrinsic Value $ 420