Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (71,905)	$ (81,836)	$ (55,748)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	2,524	2,276	1,591
Loss from valuation of convertible note payable	0	0	1,811
Stock-based compensation	7,642	12,872	15,368
Amortization of premium on investments, net	349	666	1,472
Other	600	1,043	98
Change in operating assets and liabilities:
Contracts and other receivables	1,284	8,364	(9,746)
Prepaid materials	394	(616)	(3,460)
Prepaid expenses and other assets	2,369	(778)	(683)
Accounts payable	(65)	3,123	529
Accrued liabilities	(492)	(871)	2,094
Accrued compensation	(333)	(74)	284
Deferred revenue	(72)	(1,194)	(3,090)
Deferred rent and other liabilities	(1,068)	143	(379)
Net cash used in operating activities	(58,773)	(56,882)	(49,859)
Investing activities
Purchases of short-term investments	(55,686)	(65,110)	(78,401)
Sales and maturities of short-term investments	69,941	101,387	101,306
Purchases of property and equipment	(303)	(913)	(1,363)
Acquisition of intangibles	(16)	(53)	(67)
Net cash provided by investing activities	13,936	35,311	21,475
Financing activities
Proceeds from issuance of common stock, net	43,411	641	492
Proceeds from borrowing under term loan, net	0	19,768	0
Proceeds from exercise of common stock options	4	310	6,680
Principal payments on other long-term obligations	0	(167)	(155)
Net cash provided by financing activities	43,415	20,552	7,017
Net decrease in cash and cash equivalents	(1,422)	(1,019)	(21,367)
Cash and cash equivalents at beginning of period	14,941	15,960	37,327
Cash and cash equivalents at end of period	13,519	14,941	15,960
Supplemental disclosure of cash flow information
Net changes in restricted cash	0	(1,256)	1,256
Interest paid	(1,944)	(981)	(27)
Income taxes paid	(1)	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	0	6,653	1,656
Amounts accrued for property and equipment	11	292	223
Amounts accrued for patent expenditures	$ 0	$ 0	$ 28