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Assumptions Used to Estimate Fair Value on Grant Date, Employee Stock Purchase Plan (Detail) (Employee Stock Purchase Plan)
9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Employee Stock Purchase Plan
   
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]    
Risk-free interest rate 0.19% 0.04%
Expected volatility 36.50% 25.00%
Weighted average expected life in years 1 year 1 year
Dividend yield 3.60% 3.70%