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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
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding at January 1, 2018, Number of Shares 29,250  
Granted, Number of Shares 0 5,000
Exercised, Number of Shares (3,500) 0
Forfeited, Number of Shares 0  
Outstanding (vested and expected to vest) at September 30, 2018, Number of Shares 25,750  
Exercisable at September 30, 2018, Number of Shares 21,625  
Outstanding at January 1, 2018, Weighted Average Exercise Price per Share $ 56.84  
Granted, Weighted Average Exercise Price per Share 0.00  
Exercised, Weighted Average Exercise Price per Share 63.56  
Forfeited, Weighted Average Exercise Price per Share 0.00  
Outstanding (vested and expected to vest) at September 30, 2018, Weighted Average Exercise Price per Share 55.92  
Exercisable at September 30, 2018, Weighted Average Exercise Price per Share $ 57.80  
Outstanding (vested and expected to vest) at September 30, 2018, Weighted Average Remaining Contractual Term 6 years 7 months 6 days  
Exercisable at September 30, 2018, Weighted Average Remaining Contractual Term 6 years 3 months 18 days  
Outstanding (vested and expected to vest) at September 30, 2018, Aggregate Intrinsic Value $ 383  
Exercisable at September 30, 2018, Aggregate Intrinsic Value $ 283