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])
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Nonqualified Stock Options [Member]
   
Stockholders Equity Note [Line Items]    
Expected life (in years) 7 years 6 months 7 years 6 months
Risk-free interest rate 1.38% 1.63%
Expected volatility 24.85% 25.06%
Expected dividend yield 2.75% 2.77%