v3.21.2

Summary of significant accounting policies (Schedule of Assumptions Used to Value Convertible Notes) (Details) - Warrant [Member] |  | 12 Months Ended
--- | --- | ---
Apr. 05, 2017 $ / shares | Dec. 31, 2020 item $ / shares | Dec. 31, 2019 $ / shares | Dec. 31, 2018 $ / shares | Dec. 31, 2017 $ / shares
Market price per share (USD/share) | $ 2.75 | $ 1.45 | $ 0.95 | $ 0.85 | $ 1.06
Warrants and Rights Outstanding, Measurement Input | 2.75 | 1.00 | 1.00 | 1.00 | 1.00
Measurement Input, Risk Free Interest Rate [Member] |  |  |  |  |
Risk free rate | 1.83% | 0.07% | 1.59% | 2.53% | 2.00%
Measurement Input, Expected Dividend Rate [Member] |  |  |  |  |
Warrants and Rights Outstanding, Measurement Input | item |  | 0.00 |  |  |
Measurement Input, Expected Term [Member] |  |  |  |  |
Expected term | 4 years | 3 months 4 days | 1 year 3 months 4 days | 2 years 3 months 4 days | 3 years 3 months 4 days
Measurement Input, Price Volatility [Member] |  |  |  |  |
Expected volatility | 106.46% | 64.06% | 78.11% | 81.54% | 110.46%

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Price of a single share of a number of saleable stocks of a company. + References No definition available. + Details
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- Definition Value of input used to measure outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Period between issuance and expiration of outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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