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Schedule of Weighted Average Stock Options Exercise Price (Detail)
12 Months Ended
Dec. 31, 2018
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award  
Outstanding at January 1 $ 13.92
Options granted 47.70
Options exercised 10.89
Outstanding at December 31 19.06
Exercisable at December 31 $ 8.31