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Stock Plans (Tables) | 6 Months Ended
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Jun. 30, 2018
Share-based Compensation [Abstract] |
Share-based Compensation, Stock Options, Activity [Table Text Block] | The following table summarizes the stock-based compensation expense recorded for awards under the stock option plans for the three and six-month periods ended June 30, 2018 and 2017:
|  | Three Months Ended ‎ June 30, |  |  | Six Months Ended ‎ June 30, |
(in thousands, except per share amounts) |  | 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |
Research and development |  | $ | 156 |  |  | $ | 128 |  |  | $ | 318 |  |  | $ | 245 |
General and administrative |  |  | 246 |  |  |  | 254 |  |  |  | 516 |  |  |  | 558 |
Total stock-based compensation expenses |  | $ | 402 |  |  | $ | 382 |  |  | $ | 834 |  |  | $ | 803 |
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] | We use the Black-Scholes-Merton option-pricing model with the following assumptions to estimate the stock-based compensation expense for the three and six-month periods ended June 30, 2018 and 2017:
|  | Three Months Ended ‎ June 30, |  |  | Six Months Ended ‎ June 30, |
|  | 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |
Weighted-average risk-free interest rate |  |  | 2.9 | % |  |  | 2.0 | % |  |  | 2.7 | % |  |  | 2.1 | %
Expected dividend payments |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Expected holding period (years) 1 |  |  | 6.4 |  |  |  | 6.5 |  |  |  | 6.4 |  |  |  | 6.5 |
Weighted-average volatility factor 2 |  |  | 0.91 |  |  |  | 0.88 |  |  |  | 0.89 |  |  |  | 0.88 |
Estimated forfeiture rates 3 |  |  | 25 | % |  |  | 27 | % |  |  | 26 | % |  |  | 28 | %
| (1) | Expected holding period is based on historical experience of similar awards, giving consideration to the contractual terms of the stock-based awards, vesting schedules and the expectations of future employee behavior.
| (2) | Weighted average volatility is based on the historical volatility of our common stock.
| (3) | Estimated forfeiture rates are based on historical data.
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] | The following table summarizes option activity for the six-month period ended June 30, 2018:
(in thousands, except per share amounts) |  | Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Option Term |  |  | Aggregate Intrinsic Value |
Outstanding at January 1, 2018 |  |  | 2,728 |  |  | $ | 4.32 |  |  |  | 5.75 |  |  | $ | 30 |
Granted |  |  | 950 |  |  |  | 0.97 |  |  |  |  |  |  |  |  |
Exercised |  |  | — |  |  |  | — |  |  |  |  |  |  |  |  |
Expired |  |  | (18 | ) |  |  | 8.36 |  |  |  |  |  |  |  |  |
Cancelled or forfeited |  |  | (12 | ) |  |  | 5.46 |  |  |  |  |  |  |  |  |
Outstanding at June 30, 2018 |  |  | 3,648 |  |  | $ | 3.42 |  |  |  | 6.44 |  |  | $ | 115 |
Exercisable at June 30, 2018 |  |  | 2,796 |  |  | $ | 4.00 |  |  |  | 5.88 |  |  | $ | 57 |

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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited during the year. + 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&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 employee stock purchase plans, including, but not limited to: (a) expected term, (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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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