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Assumptions Used in Black-Scholes Model (Detail)
3 Months Ended9 Months Ended
Sep. 30, 2011
Entity
Year
Sep. 30, 2010
Year
Sep. 30, 2011
Year
Entity
Sep. 30, 2010
Year
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]    
Expected term (in years)5.755.755.755.75
Expected volatility27.70%27.62%27.41%28.39%
Expected dividend yield0.00%0.00%0.00%0.00%
Risk free interest rate1.38%1.71%2.23%2.38%