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Stock-based compensation (Tables) | 6 Months Ended | 12 Months Ended
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Jun. 30, 2018 | Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |
Summary of stock option activity and related information | A summary of our stock option activity and related information as of June 30, 2018 is as follows:
| Shares |  | Weighted average exercise price per share |  | Weighted average remaining contractual term (years) |  | Aggregate intrinsic value (in thousands)
Outstanding at December 31, 2017 | 5,745,815 |  |  | $ | 7.28 |  |  | 8.32 |  | $ | 43 |
Granted | 1,179,877 |  |  | 0.96 |  |  |  |  |  |
Forfeited/Canceled | (1,529,717 | ) |  | 5.90 |  |  |  |  |  |
Outstanding at June 30 | 5,395,975 |  |  | 6.28 |  |  | 8.05 |  | 1 |
Exercisable at June 30, 2018 | 2,576,138 |  |  | 10.88 |  |  | 6.96 |  | 1 |
| A summary of our stock option activity and related information is as follows:
| Shares |  | Weighted average exercise price per share |  | Weighted average remaining contractual term (years) |  | Aggregate intrinsic value (in thousands)
Outstanding at December 31, 2016 | 4,611,392 |  |  | $ | 14.42 |  |  | 8.23 |  | $ | 45 |
Granted | 4,386,856 |  |  | 1.51 |  |  |  |  |
Forfeited / Canceled | (3,252,433 | ) |  | 9.63 |  |  |  |  |
Outstanding at December 31, 2017 | 5,745,815 |  |  | 7.28 |  |  | 8.32 |  | 43 |
Exercisable at December 31, 2017 | 2,370,335 |  |  | 13.37 |  |  | 7.08 |  | 42 |
Vested and expected to vest at December 31, 2017 | 5,745,815 |  |  | 7.28 |  |  | 8.32 |  | 43 |
Schedule of assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model | The fair value of each stock-based option award is estimated on the grant date using the Black-Scholes option pricing model with the following assumptions:
| Three months ended June 30, |  | Six months ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Risk-free interest rate | — |  | 1.3% - 2.0% |  | 2.7% |  | 1.3% - 2.2%
Dividend yield | — |  | — |  | — |  | —
Volatility | — |  | 89% - 109% |  | 83% - 85% |  | 89% - 109%
Expected term (years) |  |  | 1.8 - 6.9 |  | 6.1 |  | 1.8 - 6.9
| The fair value of each stock option is estimated using the Black-Scholes option pricing model using the following assumptions:
| December 31,
| 2017 |  | 2016 |  | 2015
Risk-free interest rate | 1.3%-2.2% |  | 1.3%-2.0% |  | 1.6%-2.3%
Dividend yield | — |  | — |  | —
Volatility | 87%-109% |  | 78%-89% |  | 72%-78%
Expected term (years) | 2.0-6.9 |  | 5.3-9.9 |  | 5.3-9.9

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -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 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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