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Fair Values of Grants Estimated Using Weighted Average Assumptions (Detail) (Stock Option)
12 Months Ended
Dec. 31, 2011
Stock Option
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Volatility 27.96%
Expected dividend yield 3.24%
Expected term (in years) 6 years 2 months 5 days
Risk-free rate 1.50%