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Stock Options - Fair value weighted average assumptions (Details) (Stock options)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.17%	2.78%	1.20%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	0.68%	0.68%	0.79%
Expected term (in years)	7 years	5 years	3 years

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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