Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Employee Stock Options Granted as Determined by Black-Scholes Valuation Model Assumptions (Detail)

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Fair Value of Employee Stock Options Granted as Determined by Black-Scholes Valuation Model Assumptions (Detail) - Nonqualified Stock Options [Member]
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Stockholders Equity Note [Line Items]    
Expected life (in years) 7 years 6 months 7 years 6 months
Risk-free interest rate 2.07% 2.40%
Expected volatility 20.61% 24.26%
Expected dividend yield 2.63% 2.56%