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Summary of Nonvested Restricted Shares Activity (Detail) - Restricted Stock Units (RSUs)
12 Months Ended
Dec. 31, 2016
$ / shares
shares
NONVESTED NUMBER OF SHARES  
Nonvested awards outstanding, beginning balance | shares 181,900
Granted | shares 148,500
Vested | shares (128,100)
Forfeited | shares (3,900)
Nonvested awards outstanding, ending balance | shares 198,400
GRANT DATE FAIR VALUE  
Nonvested awards outstanding, beginning balance | $ / shares $ 58.79 [1]
Granted | $ / shares 49.53 [1]
Vested | $ / shares 53.62 [1]
Forfeited | $ / shares 54.71 [1]
Nonvested awards outstanding, ending balance | $ / shares $ 55.27 [1]
[1] Weighted Average