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Stock options' weighted average assumptions and grant date fair values (Details) - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate (%) 1.75% 2.10%
Dividend yield
Volatility factor 79.91% 92.22%
Expected option life (years) 5 years 7 months 6 days 4 years 9 months 18 days
Weighted average grant date fair value $ 1.93 $ 1.58