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Stock-Based Compensation (Details 4)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Assumptions used for computing share-based employee compensation expense      
Risk-free interest rate, Minimum 1.74% 1.97% 2.62%
Risk-free interest rate, Maximum 1.95% 3.61% 4.00%
Dividend yield 2.00% 2.00% 2.00%
Stock price volatility, Minimum 21.71% 25.26% 27.28%
Stock price volatility, Maximum 38.75% 38.43% 38.61%
Expected term 10 years 10 years 10 years