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Schedule of Weighted-Average Restricted Stock Unit Award Fair Value (Detail) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award      
Non-vested at January 1 $ 21.71 $ 18.27 $ 17.66
Awarded 45.99 28.91 18.95
Vested 20.20 17.90 17.36
Forfeited 24.67 20.35 17.80
Non-vested at December 31 $ 29.22 $ 21.71 $ 18.27