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Stock Incentive Plans (Details) (USD $)	12 Months Ended
Stock Incentive Plans (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair value of each option grant estimated on weighted-average assumptions:
Dividend yield(1)	5.74%	6.28%	7.35%
Expected volatility	34.80%	34.08%	29.40%
Risk-free interest rate	2.87%	3.23%	2.33%
Expected life (in years)	7 years 0 months	7 years 0 months	7 years 0 months
Weighted-average fair value(1)	$ 9.60	$ 7.82	$ 4.38

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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