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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 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Volatility 37.91% 35.89% 37.95% 35.89%
Expected term (in years) 5 years 5 years 5 years 5 years
Dividend yield 1.97% 1.21% 2.00% 1.17%
Risk-free interest rate 1.35% 1.76% 1.37% 1.76%