Annual report pursuant to Section 13 and 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) (USD $)
12 Months Ended
Dec. 31, 2011
Year
Dec. 31, 2010
Year
Dec. 31, 2009
Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividend yield 2.77% 2.70% 3.25%
Risk-free interest rate 2.90% 3.30% 3.22%
Expected life in years 7.5 7.5 7.5
Expected volatility 24.26% 23.59% 23.16%
Weighted average fair value of options granted $ 15.92 $ 14.83 $ 10.86