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Share-Based Compensation - Activity in Company's Plan (Detail) - 1999 Stock Option Plan [Member]
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Outstanding at January 1, 2017, Number of Shares | shares 5,550
Exercised, Number of Shares | shares (4,800)
Forfeited, Number of Shares | shares 0
Outstanding (exercisable and vested) at December 31, 2017, Number of Shares | shares 750
Outstanding at January 1, 2017, Weighted Average Exercise Price per Share | $ / shares $ 48.35
Exercised, Weighted Average Exercise Price per Share | $ / shares 49.79
Forfeited, Weighted Average Exercise Price per Share | $ / shares 0.00
Outstanding (exercisable and vested) at December 31, 2017, Weighted Average Exercise Price per Share | $ / shares $ 39.10
Outstanding (exercisable and vested), Weighted Average Remaining Contractual Term 1 year 9 months 18 days
Outstanding (exercisable and vested) Aggregate Intrinsic Value | $ $ 24