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Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Expected dividend yield 3.00% 3.00% 3.50%
Expected risk-free interest rate 1.80% 1.80% 1.20%
Volatility 28.20% 28.90% 29.60%
Expected life (in years) 5 years 6 months 5 years 6 months 6 years