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Schedule of Weighted-Average Restricted Stock Unit Award Fair Value (Detail) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award      
Non-vested at January 1 $ 17.24 $ 15.60 $ 12.72
Awarded 17.36 18.41 16.62
Vested 16.85 14.38 10.40
Forfeited 17.08 16.97 15.48
Non-vested at December 31 $ 17.46 $ 17.24 $ 15.60