XML 23 R79.htm IDEA: XBRL DOCUMENT

Stock-based compensation (Details)	12 Months Ended
Stock-based compensation (Details)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Risk free interest rate	0.26%	1.33%	0.54%
Expected option life (years)	1 year 6 months	3 years 4 months 24 days	1 year 7 months 6 days
Expected volatility	43.43%	68.18%	56.97%
Expected dividend yield	3.78%	2.89%	2.49%

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.

+ References+ Details