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Summary of Nonvested Restricted Shares Activity (Detail) - Restricted Stock Units (RSUs)
12 Months Ended
Dec. 31, 2015
$ / shares
shares
NONVESTED NUMBER OF SHARES  
Nonvested awards outstanding, beginning balance | shares 185,700
Granted | shares 112,000
Vested | shares (115,800)
Nonvested awards outstanding, ending balance | shares 181,900
GRANT DATE FAIR VALUE  
Nonvested awards outstanding, beginning balance | $ / shares $ 51.60 [1]
Granted | $ / shares 63.39 [1]
Vested | $ / shares 51.70 [1]
Nonvested awards outstanding, ending balance | $ / shares $ 58.79 [1]
[1] Weighted Average