Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net loss	$ (29,873)	$ (41,629)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	1,121	1,357
Stock-based compensation	2,984	5,247
Amortization of premium on investments, net	87	181
Other	109	190
Change in operating assets and liabilities:
Contracts and other receivables	189	967
Prepaid materials	128	(597)
Prepaid expenses and other assets	231	1,106
Accounts payable	(276)	(907)
Accrued liabilities	(927)	(814)
Accrued compensation	(761)	(885)
Contract liabilities	(36)	(36)
Deferred rent and other liabilities	(787)	(187)
Net cash used in operating activities	(27,811)	(36,007)
Investing activities
Purchases of short-term investments	0	(4,453)
Sales and maturities of short-term investments	27,151	37,929
Purchases of property and equipment	0	(124)
Acquisition of intangibles	0	(16)
Net cash provided by investing activities	27,151	33,336
Financing activities
Proceeds from issuance of common stock, net	208	265
Proceeds from exercise of common stock options	1	3
Proceeds from capital lease financing	417	0
Net cash provided by financing activities	626	268
Net decrease in cash and cash equivalents	(34)	(2,403)
Cash and cash equivalents at beginning of period	13,519	14,941
Cash and cash equivalents at end of period	13,485	12,538
Supplemental disclosure of cash flow information
Interest paid	(1,045)	(946)
Income taxes paid	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Non-cash acquisition of property and equipment	306	0
Amounts accrued for property and equipment	$ 22	$ 110