Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net loss	$ (3,260)	$ (16,025)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	502	563
Stock-based compensation	959	1,627
Amortization of premium on investments, net	0	47
Gain on reduction of lease liability	1,317	0
Other	139	93
Change in operating assets and liabilities:
Contracts and other receivables	(2,766)	185
Prepaid materials	368	128
Prepaid expenses and other assets	532	26
Accounts payable	936	(1,347)
Accrued liabilities	120	377
Accrued compensation	314	(868)
Operating lease liabilities	0	0
Contract liabilities	(2,518)	(18)
Deferred rent and other liabilities	0	(300)
Other liabilities	(1,076)	(90)
Net cash used in operating activities	(4,433)	(15,602)
Investing activities
Sales and maturities of short-term investments	0	11,439
Sales and disposals of property and equipment	161	0
Net cash provided by investing activities	161	11,439
Financing activities
Proceeds from issuance of common stock, net	2,086	208
Proceeds from exercise of common stock options	0	1
Principal payments on term loan	(1,364)	0
Proceeds from capital lease financing	0	492
Payments on financing leases	(65)	0
Net cash provided by financing activities	657	701
Net decrease in cash and cash equivalents	(3,615)	(3,462)
Cash and cash equivalents at beginning of period	13,935	13,519
Cash and cash equivalents at end of period	10,320	10,057
Supplemental disclosure of cash flow information
Interest paid	(437)	(506)
Income taxes paid	0	(1)
Supplemental disclosure of non-cash investing and financing activities
Non-cash acquisition of property and equipment	$ 0	$ 191