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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, 2018
Jun. 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 (1,500) 0
Forfeited, Number of Shares 0  
Outstanding (vested and expected to vest) at June 30, 2018, Number of Shares 27,750  
Exercisable at June 30, 2018, Number of Shares 23,125  
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 52.81  
Forfeited, Weighted Average Exercise Price per Share 0.00  
Outstanding (vested and expected to vest) at June 30, 2018, Weighted Average Exercise Price per Share 57.05  
Exercisable at June 30, 2018, Weighted Average Exercise Price per Share $ 59.44  
Outstanding (vested and expected to vest) at June 30, 2018, Weighted Average Remaining Contractual Term 6 years 8 months 12 days  
Exercisable at June 30, 2018, Weighted Average Remaining Contractual Term 6 years 1 month 6 days  
Outstanding (vested and expected to vest) at June 30, 2018, Aggregate Intrinsic Value $ 880  
Exercisable at June 30, 2018, Aggregate Intrinsic Value $ 678