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Stock based compensation (Details)	12 Months Ended
Stock based compensation (Details)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Weighted-average expected life (years)	5 years 8 months 12 days	4 years	4 years 6 months 22 days
Expected volatility rate	62.50%	63.60%	60.50%
Expected dividend yield	0.00%	0.00%	0.00%
Risk-free interest rate	0.95%	0.83%	1.43%
Forfeiture rate	0.00%	0.00%	0.00%

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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