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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at January 1, 2017, Number of Shares 52,750    
Granted, Number of Shares 5,000 0 10,500
Exercised, Number of Shares (28,500) (500) 0
Forfeited, Number of Shares 0    
Outstanding (vested and expected to vest) at December 31, 2017, Number of Shares 29,250    
Exercisable at December 31, 2017, Number of Shares 21,625    
Outstanding at January 1, 2017, Weighted Average Exercise Price per Share $ 54.54    
Granted, Weighted Average Exercise Price per Share 48.00    
Exercised, Weighted Average Exercise Price per Share 46.90    
Forfeited, Weighted Average Exercise Price per Share 0.00    
Outstanding (vested and expected to vest) at December 31, 2017, Weighted Average Exercise Price per Share 56.84    
Exercisable at December 31, 2017, Weighted Average Exercise Price per Share $ 60.69    
Outstanding (vested and expected to vest) at December 31, 2017, Weighted Average Remaining Contractual Term 7 years 1 month 6 days    
Exercisable at December 31, 2017, Weighted Average Remaining Contractual Term 6 years 6 months    
Outstanding (vested and expected to vest) at December 31, 2017, Aggregate Intrinsic Value $ 423    
Exercisable at December 31, 2017, Aggregate Intrinsic Value $ 231