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Share incentive plan (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Risk-free interest rate, Minimum 1.18% 1.30% 1.55%
Risk-free interest rate ,Maximum 2.06% 1.90% 2.21%
Expected price volatility, Maninum 79.34% 70.40% 69.49%
Expected price volatility, Maximum 90.65% 83.74% 84.63%
Dividend yield 0.00% 0.00% 0.00%
Minimum [Member]      
Expected option term (years) 5 years 5 years 1 month 13 days 5 years 1 month 13 days
Maximum [Member]      
Expected option term (years) 7 years 7 years 7 years