Common Stock (Details) (USD $)	12 Months Ended
Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Assumptions used in the pricing model and the weighted average grant-date fair value of stock options granted
Expected volatility	17.50%	17.40%	15.60%
Expected term (in years)	5.0	5.0	5.0
Interest rate	2.30%	2.40%	1.90%
Dividend yield	4.80%	5.60%	5.40%
Weighted average grant-date fair value	$ 3.23	$ 2.23	$ 1.80

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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