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Stock-based compensation - Assumptions Used to Estimate Weighted Average Fair Value of Options (Details) (Stock options)	3 Months Ended		6 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	73.00%	65.00%	73.00%	63.00%
Expected term (in years)	4 years 9 months 4 days	4 years 6 months	4 years 9 months 18 days	4 years 6 months
Risk-free interest rate	0.71%	0.96%	0.69%	0.96%
Dividend yield	0.00%	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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