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Stock-based compensation - Stock options (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Shares |  |  |  |  |  |  |  |
Outstanding at the beginning of the period (in shares) |  |  |  | 5,745,815 | 4,611,392 | 4,611,392 |  |
Granted (in shares) |  | 1,179,877 | 2,183,106 |  | 4,015,356 | 4,386,856 |  |
Forfeited/Canceled (in shares) |  |  |  |  |  | (3,252,433) |  |
Outstanding at the end of the period (in shares) |  |  |  |  |  | 5,745,815 | 4,611,392 |
Exercisable at the end of the period (in shares) |  |  |  |  |  | 2,370,335 |  |
Weighted average exercise price per share |  |  |  |  |  |  |  |
Outstanding at the beginning of the period (in USD per share) |  |  |  | $ 7.28 | $ 14.42 | $ 14.42 |  |
Granted (in dollars per share) |  | $ 0.96 | $ 1.46 |  | 1.51 | 1.51 |  |
Forfeited/Canceled (in dollars per share) |  |  |  |  |  | 9.63 |  |
Outstanding at the end of period (in USD per share) |  |  |  |  |  | 7.28 | $ 14.42 |
Exercisable at the end of the period (in USD per share) |  |  |  |  |  | $ 13.37 |  |
Weighted average remaining contractual term (years) |  |  |  |  |  |  |  |
Outstanding |  |  |  |  |  | 8 years 3 months 26 days | 8 years 2 months 23 days |
Exercisable at the end of the period |  |  |  |  |  | 7 years 29 days |  |
Aggregate intrinsic value |  |  |  |  |  |  |  |
Outstanding |  |  |  |  |  | $ 43,000 | $ 45,000 |
Exercisable at the end of period |  |  |  |  |  | $ 42,000 |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Dividend yield |  |  |  |  |  | 0.00% | 0.00% | 0.00%
Expected term (years) |  |  |  |  |  |  |  |
Additional disclosures |  |  |  |  |  |  |  |
Total unrecognized compensation cost |  |  |  |  |  | $ 4,200,000 |  |
Weighted average period for recognition of compensation cost |  |  |  |  |  | 2 years 7 months 17 days |  |
Weighted average grant date fair value (in dollars per share) |  | $ 0.70 | $ 1.11 |  | $ 1.15 |  |  |
Non-Employee Stock |  |  |  |  |  |  |  |
Shares |  |  |  |  |  |  |  |
Granted (in shares) |  |  | 150,000 |  | 150,000 |  |  |
Weighted average exercise price per share |  |  |  |  |  |  |  |
Granted (in dollars per share) |  |  | $ 1.60 |  | $ 1.60 |  |  |
Minimum [Member] |  |  |  |  |  |  |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  |  | 1.30% | 1.30% | 1.60%
Volatility |  |  |  |  |  | 87.00% | 78.00% | 72.00%
Expected term (years) |  |  |  |  |  | 2 years | 5 years 3 months 18 days | 5 years 3 months 18 days
Maximum [Member] |  |  |  |  |  |  |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Risk-free interest rate |  |  |  |  |  | 2.20% | 2.00% | 2.30%
Volatility |  |  |  |  |  | 109.00% | 89.00% | 78.00%
Expected term (years) |  |  |  |  |  | 6 years 10 months 25 days | 9 years 10 months 24 days | 9 years 10 months 24 days
Stock options |  |  |  |  |  |  |  |
Shares |  |  |  |  |  |  |  |
Outstanding at the beginning of the period (in shares) |  |  |  | 5,745,815 |  |  |  |
Granted (in shares) | 143,000 |  |  | 1,179,877 |  |  |  |
Forfeited/Canceled (in shares) |  |  |  | (1,529,717) |  |  |  |
Outstanding at the end of the period (in shares) |  | 5,395,975 |  | 5,395,975 |  | 5,745,815 |  |
Exercisable at the end of the period (in shares) |  | 2,576,138 |  | 2,576,138 |  |  |  |
Weighted average exercise price per share |  |  |  |  |  |  |  |
Outstanding at the beginning of the period (in USD per share) |  |  |  | $ 7.28 |  |  |  |
Granted (in dollars per share) |  |  |  | 0.96 |  |  |  |
Forfeited/Canceled (in dollars per share) |  |  |  | 5.90 |  |  |  |
Outstanding at the end of period (in USD per share) |  | $ 6.28 |  | 6.28 |  | $ 7.28 |  |
Exercisable at the end of the period (in USD per share) |  | $ 10.88 |  | $ 10.88 |  |  |  |
Weighted average remaining contractual term (years) |  |  |  |  |  |  |  |
Outstanding |  |  |  | 8 years 18 days |  | 8 years 3 months 25 days |  |
Exercisable at the end of the period |  |  |  | 6 years 11 months 15 days |  |  |  |
Aggregate intrinsic value |  |  |  |  |  |  |  |
Outstanding |  | $ 1,000 |  | $ 1,000 |  | $ 43,000 |  |
Exercisable at the end of period |  | $ 1,000 |  | $ 1,000 |  |  |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Risk-free interest rate |  | 0.00% |  | 2.70% |  |  |  |
Dividend yield |  | 0.00% | 0.00% | 0.00% | 0.00% |  |  |
Volatility |  | 0.00% |  |  |  |  |  |
Expected term (years) |  |  |  | 6 years 1 month 9 days |  |  |  |
Additional disclosures |  |  |  |  |  |  |  |
Stock options exercised |  | $ 0 | $ 0 |  |  |  |  |
Total unrecognized compensation cost |  | $ 2,800,000 |  | $ 2,800,000 |  |  |  |
Weighted average period for recognition of compensation cost |  |  |  | 2 years 10 months 24 days |  |  |  |
Stock options | Minimum [Member] |  |  |  |  |  |  |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 1.30% |  | 1.30% |  |  |
Volatility |  |  | 89.00% | 83.00% | 89.00% |  |  |
Expected term (years) |  |  | 1 year 9 months 18 days |  | 1 year 9 months 18 days |  |  |
Stock options | Maximum [Member] |  |  |  |  |  |  |  |
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 2.00% |  | 2.20% |  |  |
Volatility |  |  | 109.00% | 85.00% | 109.00% |  |  |
Expected term (years) |  |  | 6 years 10 months 24 days |  | 6 years 10 months 24 days |  |  |

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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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://www.xbrl.org/2003/role/presentationRef -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=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3)-(4) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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- Definition Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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