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Weighted-Average Assumptions used to Estimate Fair Value of Options Granted (Detail) (Stock Options)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Stock Options
     
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Weighted Average Fair Values [Line Items]      
Risk-free interest rate 0.81% 1.92% 2.12%
Expected life of option 4 years 10 months 6 days 4 years 8 months 12 days 4 years 7 months 28 days
Expected dividend yield 2.28% 2.34% 2.22%
Expected volatility 28.46% 26.82% 26.42%