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Schedule of Weighted-Average Restricted Stock Unit Award Fair Value (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award      
Non-vested at January 1 $ 18.27 $ 17.66 $ 17.35
Awarded 28.91 18.95 17.66
Vested 17.90 17.36 16.93
Forfeited 20.35 17.80 17.28
Non-vested at December 31 $ 21.71 $ 18.27 $ 17.66