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Fair value of financial instruments and investments - Fair Value of Liabilities (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Warrants |  |
Changes in the fair value of warrant liability |  |
Beginning balance | $ 1 | $ 48
Fair value of issuances | 0 | 0
Change in fair value | 0 | (47)
Payments | 0 |
Ending balance | $ 1 | $ 1
Warrants | Minimum |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Volatility | 69.00% | 62.00%
Risk-free interest rate | 1.89% | 0.62%
Strike price (in dollars per share) | $ 128.00 | $ 128.00
Expected life | 1 year 7 months 2 days | 5 months 12 days
Warrants | Maximum |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Volatility | 69.00% | 67.00%
Risk-free interest rate | 1.89% | 1.34%
Strike price (in dollars per share) | $ 2,520.00 | $ 2,520.00
Expected life | 1 year 8 months 19 days | 2 years 8 months 19 days
Warrants | Common stock |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Fair value of shares (in dollars per share) | $ 16.68 | $ 10.91
Stock Appreciation Rights (SARs) |  |
Changes in the fair value of warrant liability |  |
Beginning balance | $ 865 | $ 0
Fair value of issuances | 0 | 140
Change in fair value | 1,864 | 725
Payments | (1,064) |
Ending balance | $ 1,665 | $ 865
Stock Appreciation Rights (SARs) | Minimum |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Volatility | 31.00% | 48.00%
Risk-free interest rate | 1.28% | 0.44%
Strike price (in dollars per share) | $ 6.76 | $ 6.76
Expected life | 0 days | 0 days
Stock Appreciation Rights (SARs) | Maximum |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Volatility | 70.00% | 71.00%
Risk-free interest rate | 1.89% | 1.47%
Strike price (in dollars per share) | $ 30.86 | $ 30.86
Expected life | 2 years | 3 years
Stock Appreciation Rights (SARs) | Common stock |  |
Assumption used to estimate the fair value of warrant liability by utilizing the Black-Scholes option-pricing model |  |
Fair value of shares (in dollars per share) | $ 16.68 | $ 10.91

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- Definition Agreed upon price for the exchange of the underlying asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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