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Share-Based Compensation - Activity in Company's Plan - Long Term Incentive Plan (Detail) - Long Term Incentive Plan Stock Option [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at January 1, 2020, Number of Shares 31,750    
Granted, Number of Shares 25,500 5,000 5,000
Exercised, Number of Shares (5,050) (4,000) (3,500)
Forfeited, Number of Shares (1,250)    
Outstanding (vested and expected to vest) at December 31, 2020, Number of Shares 50,950    
Exercisable at December 31, 2020, Number of Shares 21,700    
Outstanding at January 1, 2020, Weighted Average Exercise Price per Share $ 58.77    
Granted, Weighted Average Exercise Price per Share 48.13    
Exercised, Weighted Average Exercise Price per Share 47.68    
Forfeited, Weighted Average Exercise Price per Share 48.00    
Outstanding (vested and expected to vest) at December 31, 2020, Weighted Average Exercise Price per Share 54.81    
Exercisable at December 31, 2020, Weighted Average Exercise Price per Share $ 61.77    
Outstanding (vested and expected to vest) at December 31, 2020, Weighted Average Remaining Contractual Term 7 years 6 months    
Exercisable at December 31, 2020, Weighted Average Remaining Contractual Term 5 years    
Outstanding (vested and expected to vest) at December 31, 2020, Aggregate Intrinsic Value $ 726    
Exercisable at December 31, 2020, Aggregate Intrinsic Value $ 151