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Stock Incentive Plans (Details) (USD $)
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Fair value of each option grant estimated on weighted-average assumptions:    
Dividend yield 5.74% 6.28%
Expected volatility 34.80% 34.08%
Risk-free interest rate 2.87% 3.23%
Expected life (in years) 7.0 7.0
Weighted-average fair value $ 9.60 $ 7.82