Long-term debt - Senior Secured Credit facility (Details)							1 Months Ended		12 Months Ended
	Nov. 08, 2018 $ / shares shares	Nov. 05, 2018 USD ($) item $ / shares shares	Oct. 26, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 01, 2017 USD ($) item $ / shares	May 11, 2016 USD ($) item installment	Jun. 30, 2017 USD ($) $ / shares shares	Feb. 28, 2017 $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Long-term debt
Proceeds from debt											$ 58,419,000
Debt discount amortization									$ 961,000	$ 1,316,000	406,000
Derivative liability									2,017,000	1,660,000
Unamortized discount on debt									$ 3,334,000
Recognition of beneficial conversion feature on convertible notes										$ 613,000
Payment of senior debt and fee											26,063,000
Senior Notes and Subordinated Debt
Long-term debt
Repayments of debt						$ 33,000,000
Senior debt (Hercules LSA)
Long-term debt
Interest paid						100,000
Payments of end of term fee						1,100,000
Debt discount amortization											104,000
Payment of senior debt and fee						24,300,000
Prepayment penalty						243,000
Senior Secured Convertible Note
Long-term debt
Face amount of debt issued					$ 6,600,000
Interest rate (as a percent)					12.95%
Term of debt (in years)					1 year
Effective interest rate during debt term (as a percent)		16.69%							25.35%
Time period before debt maturity when debt conversion option expires					5 days
Increase in prepayment fee (in basis points)					300
Conversion price, expressed as a percentage of stock price					95.00%
Number of trading days used to determine conversion stock price					3 days
Unamortized discount on debt					$ 600,000
Recognition of beneficial conversion feature on convertible notes					$ 600,000
Maximum percentage of stock for note holder					9.985%
Maximum percentage of stock for aggregate conversion					19.90%
Common Stock
Long-term debt
Issuance of common stock, net of issuance costs (in shares) | shares									20,651,524	12,019,639
Underwritten Public Offering | Common Stock
Long-term debt
Issuance of common stock, net of issuance costs (in shares) | shares	19,999,999			4,800,000			4,800,000	5,750,000
Public offering price (in dollars per share) | $ / shares	$ 2.30			$ 6.25			$ 6.25	$ 5.00
Gross proceeds from issuance of stock				$ 30,000,000			$ 30,000,000
Minimum | Senior Secured Convertible Note
Long-term debt
Prepayment fee of debt (in percentage)					2.00%
Conversion price | $ / shares					$ 7.00
Minimum | Common Stock
Long-term debt
Gross proceeds from issuance of stock		$ 4,500,000
Minimum | Underwritten Public Offering | Common Stock
Long-term debt
Gross proceeds from issuance of stock		30,000,000
Maximum | Senior Secured Convertible Note
Long-term debt
Prepayment fee of debt (in percentage)					12.75%
Senior Secured Credit Facility | Registration Agreement
Long-term debt
Maximum reimbursable legal fees					$ 25,000
Liability recorded for expected unmet agreement obligations									$ 0
Senior Secured Credit Facility | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued						60,000,000
Proceeds from debt						$ 60,000,000
Number of equal annual installments | installment						3
Interest rate (as a percent)						12.95%
Deferred interest payments						$ 6,600,000
Number of first interest payments for which the reporting entity has the option to defer | item						4
Debt yield enhancement fee						$ 1,350,000
Debt legal costs		25,000
Term of debt (in years)						6 years
Minimum cash on deposit to maintain under debt arrangement						$ 5,000,000
Effective interest rate during debt term (as a percent)									15.03%
Debt discount amortization									$ 961,000	$ 1,316,000
Unamortized discount on debt									$ 3,334,000	$ 2,843,000
Senior Secured Credit Facility | Debt issued to entities affiliated with Deerfield | Company's Attorneys
Long-term debt
Debt legal costs						173,000
Senior Secured Credit Facility | Debt issued to entities affiliated with Deerfield | On Behalf Of Deerfield's Attorneys
Long-term debt
Debt legal costs						$ 58,000
Senior Secured Credit Facility | Deerfield Private Design Found III, LP
Long-term debt
Debt borrowing structure (as a percent)						66.67%
Senior Secured Credit Facility | Deerfield Special Situations Fund, LP
Long-term debt
Debt borrowing structure (as a percent)						33.33%
Senior Secured Credit Facility | Senior Secured Convertible Note
Long-term debt
Derivative liability					$ 2,100,000
Minimum premiums as percent required to treat as derivative					10.00%
Payment of debt modification fee					$ 40,000
Maximum time period to register shares per agreement					30 days
Maximum time period for registration to become effective per agreement					75 days
Senior Secured Credit Facility | Minimum | Senior Secured Convertible Note
Long-term debt
Conversion price | $ / shares					$ 7.00
Senior Secured Credit Facility | Maximum | Senior Secured Convertible Note
Long-term debt
Number of shares which may be issued for conversion of convertible notes | item					940,924
Second Amendment Facility Due in May 2019 | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued		$ 52,500,000
Prepayment of premium amount (as a percent)		6.25%
Non-refundable exit fee		$ 750,239
Payment of senior debt and fee		$ 7,500,000
Second Amendment Facility Due in May 2019 | Common Stock | Debt issued to entities affiliated with Deerfield
Long-term debt
Share price, percentage of Last Bid Price		93.00%
Principal payment (in dollars per shares) | $ / shares		$ 3.00
Ownership Cap percentage		4.985%
Number of trading days used to determine conversion stock price		10 days
Second Amendment Facility Due in May 2019 | 2019 | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued		$ 7,500,000
Second Amendment Facility Due in May 2019 | 2020 | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued		15,000,000
Second Amendment Facility Due in May 2019 | 2021 | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued		15,000,000
Second Amendment Facility Due in May 2019 | 2022 | Debt issued to entities affiliated with Deerfield
Long-term debt
Face amount of debt issued		$ 15,000,000
Second Amendment Facility Due in May 2019 | Maximum | Common Stock | Debt issued to entities affiliated with Deerfield
Long-term debt
Number of shares issued upon conversion of debt | shares		2,135,625
Second Amendment Facility Due in May 2020 | Common Stock | Debt issued to entities affiliated with Deerfield
Long-term debt
Ownership Cap percentage		4.985%
Conversion price | $ / shares		$ 10.00
Conversion price, expressed as a percentage of stock price		95.00%
Number of trading days used to determine conversion stock price		3 days
Second Amendment Facility Due in May 2020 | Maximum | Common Stock | Debt issued to entities affiliated with Deerfield
Long-term debt
Percentage of principal amount available for conversion		50.00%
Number of shares issued upon conversion of debt | shares		3,796,668
Conversion price, expressed as a percentage of stock price		83.00%
Number of shares which may be issued for conversion of convertible notes | item		3,796,668
Essex Capital Corporation, as Investor | 10% subordinated note payable
Long-term debt
Face amount of debt issued						$ 5,900,000
Repayments of debt						5,900,000						$ 5,900,000
Interest paid						$ 1,300,000
Interest accrued											$ 263,000
Interest rate (as a percent)						10.00%			10.00%
Conversion Of Convertible Notes | Senior Secured Convertible Note
Long-term debt
Amount of debt converted into stock			$ 6,600,000
Number of shares issued upon conversion of debt | shares			929,967
Conversion price | $ / shares			$ 7.08
Conversion price, expressed as a percentage of stock price			95.00%
Number of trading days used to determine conversion stock price			3 days
Conversion Of Convertible Notes | Senior Secured Credit Facility
Long-term debt
Amount of debt converted into stock			$ 6,600,000
Number of shares issued upon conversion of debt | shares			929,967
Conversion price | $ / shares			$ 7.08
Conversion price, expressed as a percentage of stock price			95.00%
Number of trading days used to determine conversion stock price		3 days	3 days