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Debt - Term Loan (Details)			3 Months Ended		9 Months Ended
Debt - Term Loan (Details)	Jul. 31, 2018 USD ($) $ / shares shares	Jul. 24, 2015 USD ($)	Sep. 30, 2018 USD ($) item tranche $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item tranche $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt
Number of remaining tranches \| tranche			2		2
Purchase of additional common stock through warrants \| shares			993,204		993,204
Exercise price of warrant \| $ / shares			$ 10.40		$ 10.40
Term of warrants			5 years		5 years
Interest (expense) income			$ (172,000)	$ (132,000)	$ (476,000)	$ (107,000)
Minimum aggregate future payments
2018			870,000		870,000
2019			4,809,000		4,809,000
2020			4,573,000		4,573,000
2021			1,370,000		1,370,000
Total minimum payments			11,622,000		11,622,000
Amount representing interest and discounts			(1,543,000)		(1,543,000)
Present value of minimum payments			10,079,000		10,079,000
Current portion			(3,681,000)		(3,681,000)		$ (1,355,000)
Long-term portion			$ 6,398,000		$ 6,398,000		$ 8,683,000
Risk Free Interest Rate
Debt
Warrant measurement	2.96		2.0		2.0
Price Volatility
Debt
Warrant measurement	66		65.1		65.1
Expected Term
Debt
Term of warrants	10 years		5 years		5 years
Expected Dividend Rate
Debt
Warrant measurement	0		0		0
Minimum
Debt
Interest rate Percentage			4.50%		4.50%
Maximum
Debt
Interest rate Percentage			5.25%		5.25%
Term Loan
Debt
Principal amount		$ 15,000,000
Interest rate Percentage			5.25%		5.25%
Debt issue costs			$ 100,000		$ 100,000
Final payment fee (as a percentage)		10.00%
Success fee (as a percentage)		3.50%
Interest (expense) income			200,000	100,000	600,000	100,000
Term Loan \| SVB
Debt
Purchase of additional common stock through warrants \| shares	36,000
Exercise price of warrant \| $ / shares	$ 7.47
Term of warrants	10 years
Additional paid in capital upon issuance with offset of loan discount	$ 200,000
Debt discount amortized as interest expenses	10,000		1,000	1,000	1,000	1,000
Legal fees incurred			$ 20,000	$ 20,000	$ 20,000	$ 20,000
Interest (expense) income	$ 13,000
Term Loan \| Minimum
Debt
Cash security (as a percentage)	100.00%				50.00%
Term Loan \| Minimum \| SVB
Debt
Unrestricted cash	$ 18,000,000
Term Loan \| Maximum
Debt
Convertible interest rate (as a percentage)		5.00%
Term Loan \| Prior to first anniversary effective date
Debt
Prepayment fee (as a percentage)		2.00%
Term Loan \| Occurs thereafter
Debt
Prepayment fee (as a percentage)		1.00%
Initial Tranche
Debt
Principal amount		$ 10,000,000
Additional Tranche available to be drawn through July 31, 2018
Debt
Principal amount		$ 5,000,000