Loan Payable - Term loans and line of credit (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument
Long-term debt, current maturities	$ (19,009,000)		$ (19,009,000)		$ (7,927,000)
Long-term debt, excluding current maturities	9,579,000		9,579,000		20,099,000
Amortization of deferred debt issuance costs			583,000	$ 493,000
Restricted cash	758,000		758,000		758,000
Letter of credit
Debt Instrument
Letters of credit outstanding, amount	758,000		$ 758,000
Revolving credit facility
Debt Instrument
Line of credit facility, interest rate description			(a) the greater of 1.25% per annum or a base rate as determined by a three-month LIBOR-based formula, plus (b) an applicable margin between 2.95% and 3.95%
Line of credit facility, unused capacity, commitment fee percentage			0.75%
Line of credit facility, maximum borrowing capacity	5,000,000		$ 5,000,000
Line of credit facility, fair value of amount outstanding	0		0		0
Term Loans
Debt Instrument
Long-term debt, gross	35,000,000		35,000,000		35,000,000
Long-term debt	28,588,000		28,588,000		28,026,000
Final fee obligation	2,429,000		2,429,000		2,429,000
Unamortized debt issuance costs	(1,079,000)		(1,079,000)		(1,641,000)
Long-term debt, current maturities	19,009,000		19,009,000		7,927,000
Long-term debt, excluding current maturities	9,579,000		9,579,000		20,099,000
Amortization of deferred debt issuance costs	293,000	$ 342,000	$ 583,000	$ 493,000
Debt instrument, interest only period end			Jan. 01, 2019
Debt instrument, maturity date			Mar. 12, 2020
Debt instrument, interest rate terms			a) the greater of 1.00% or the three year treasury rate in effect at the time of funding, plus (b) an applicable margin between 4.95% and 5.90% per annum.
Term Loan A
Debt Instrument
Long-term debt, gross	10,000,000		$ 10,000,000		10,000,000
Long-term debt	$ 7,619,000		$ 7,619,000		7,619,000
Debt instrument, interest rate, stated percentage	6.90%		6.90%
Term Loan B
Debt Instrument
Long-term debt, gross	$ 10,000,000		$ 10,000,000		10,000,000
Long-term debt	$ 7,619,000		$ 7,619,000		7,619,000
Debt instrument, interest rate, stated percentage	6.90%		6.90%
Term Loan C
Debt Instrument
Long-term debt, gross	$ 15,000,000		$ 15,000,000		15,000,000
Long-term debt	$ 12,000,000		$ 12,000,000		$ 12,000,000
Debt instrument, interest rate, stated percentage	7.40%		7.40%