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Term Loan Obligations (Details) - USD ($) |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2018 | Aug. 31, 2018 | Jun. 19, 2018 | Dec. 29, 2017 | Nov. 28, 2017 | May 29, 2015 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | May 24, 2018 | May 23, 2018 | Sep. 20, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares callable by warrants (in shares) |  |  |  |  |  |  |  |  |  |  | 2,300,000 | 23,400,000 |
Date from which warrants are exercisable |  |  |  |  | May 18, 2018 |  |  |  |  |  |  |  |
Minimum liquidity covenant | $ 2,000,000 |  |  |  |  |  | $ 2,000,000 |  | $ 2,000,000 |  |  |  |
Net proceeds excess receives from unrestricted cash |  |  |  |  |  |  |  |  | 4,000,000 |  |  |  |
Interest expense |  |  |  |  |  |  | 515,000 | $ 423,000 | 1,922,000 | $ 2,049,000 |  |  |
Non-cash amortization |  |  |  |  |  |  | $ 200,000 | $ 100,000 | $ 600,000 | $ 700,000 |  |  |
LIBOR [Member] | Interest Rate Floor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis variable rate |  |  |  |  |  | 1.00% |  |  |  |  |  |  |
Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Origination Date |  |  |  |  |  |  | May 29, 2015 |  | May 29, 2015 | May 29, 2015 |  |  |
Original Loan Amount | $ 17,700,000 |  |  |  |  | $ 17,700,000 |  |  | $ 17,700,000 | $ 17,700,000 |  |  |
Basis variable rate |  |  |  |  |  | 7.95% |  |  |  |  |  |  |
Maturity date | Jun. 01, 2019 |  |  |  |  |  | Jun. 01, 2019 |  | Jun. 01, 2019 |  |  |  |
Fees amount associated with loan |  |  |  |  |  | $ 1,100,000 |  |  |  |  |  |  |
Number of shares callable by warrants (in shares) |  |  |  |  |  | 188 |  |  |  |  |  |  |
Warrant exercise price (in dollars per share) |  |  |  |  |  | $ 5,175 |  |  |  |  |  |  |
Date from which warrants are exercisable |  |  |  |  |  |  | Nov. 30, 2015 |  | Nov. 30, 2015 |  |  |  |
Warrant expiration date |  |  |  |  |  |  | May 29, 2025 |  | May 29, 2025 |  |  |  |
Minimum liquidity covenant | $ 2,000,000 |  |  |  |  | $ 5,000,000 |  |  | $ 2,000,000 |  |  |  | $ 1,500,000
Extended interest-only period | Jun. 01, 2020 | Dec. 31, 2018 | Dec. 01, 2018 |  |  |  | Aug. 01, 2018 |  | Aug. 01, 2018 |  |  |  |
Debt instrument, interest-only period | The Fourth Amendment also required that the Company achieve one of the following by January 31, 2019: enter into an asset sale agreement with a minimum unrestricted net cash proceeds to the Company of $4.0 million; enter into a binding agreement for the issuance and sale of its equity securities or unsecured convertible subordinated debt which would result in unrestricted gross cash proceeds of not less than $7.5 million; or enter into a merger agreement pursuant to which the obligations under the Loan and Security Agreement would be paid down to a level satisfactory to Oxford. | The Third Amendment extends the interest-only period under the Term Loan to December 31, 2018 and also requires that the Company pay to Oxford, in accordance with its pro rata share of the loans, 75% of all proceeds received (i) from the issuance and sale of unsecured subordinated convertible debt, (ii) in connection with a joint venture, collaboration or other partnering transaction, (iii) in connection with any licenses, (iv) from dividends (other than non-cash dividends from wholly owned subsidiaries) and (v) from the sale of any assets (such requirement, the “Prepayment Requirement”). | The Second Amendment extends the interest-only period under the Term Loan to December 1, 2018 if the Company receives unrestricted gross cash proceeds of at least $15 million from the sale and issuance of the Company’s equity securities on or before August 31, 2018. |  |  |  | The amendment also extended the interest-only period under the Loan and Security Agreement through August 1, 2018, as the Company successfully closed on a financing and received unrestricted net cash proceeds in excess of $5 million on or before December 29, 2017. |  |  |  |  |  |
Net proceeds excess receives from unrestricted cash |  |  |  | $ 5,000,000 |  |  |  |  |  |  |  |  |
Amendment fee | $ 350,000 | $ 50,000 | $ 250,000 |  |  |  |  |  |  |  |  |  |
Percentage of proceeds received |  | 75.00% |  |  |  |  |  |  |  |  |  |  |
Minimum amount to be paid pursuant prepayment requirement |  | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |
Debt instrument prepayment, description |  | The Prepayment Requirement does not apply to proceeds from the sale and issuance of the Company’s equity securities, other than convertible debt. The Prepayment Requirement shall apply until an aggregate principle amount of $7.0 million has been paid pursuant to the Prepayment Requirement. However, if less than $7.0 million has been paid pursuant to the Prepayment Requirement on December 31, 2018 then the Company is required to promptly make additional payments until an aggregate principal amount of $7.0 million has been paid. |  |  |  |  |  |  |  |  |  |  |
Debt instrument, covenant compliance |  |  |  |  |  |  | we were in compliance with all of the debt covenants under the Loan and Security Agreement |  |  |  |  |  |
Minimum [Member] | Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  | 8.95% |  |  |  |  |  |  |
Net proceeds excess receives from unrestricted cash | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Unrestricted gross cash proceeds required to extend interest-only period | 7,500,000 |  | $ 15,000,000 |  |  |  |  |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Unrestricted gross cash proceeds required to extend interest-only period |  |  |  |  |  |  |  |  | $ 7,500,000 |  |  |  |
Maximum [Member] | Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Original Loan Amount | $ 3,000,000 |  |  |  |  |  | $ 3,000,000 |  | $ 3,000,000 |  |  |  |

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