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Stock options' weighted average assumptions and grant date fair values (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate (%) 2.10% 2.56%
Volatility factor 92.22% 87.44%
Expected option life (years) 4 years 9 months 18 days 4 years 7 months 6 days
Weighted average grant date fair value $ 1.58 $ 0.96