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Weighted-Average Assumptions Used to Estimate Grant Date Fair Values for Stock Options Granted (Detail) - Employee Stock Option
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Volatility 35.64% 37.95% 35.89%
Expected term (in years) 5 years 5 years 5 years
Dividend yield 1.18% 2.00% 1.17%
Risk-free interest rate 1.62% 1.37% 1.76%