Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (41,629)	$ (42,296)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	1,357	928
Stock-based compensation	5,247	5,986
Amortization of premium on investments, net	181	359
Other	190	45
Change in operating assets and liabilities:
Contracts and other receivables	967	9,878
Prepaid materials	(597)	2,834
Prepaid expenses and other assets	1,106	(1,970)
Accounts payable	(907)	787
Accrued liabilities	(814)	(1,063)
Accrued compensation	(885)	(529)
Deferred revenue	(36)	(972)
Deferred rent and other liabilities	(187)	(262)
Net cash used in operating activities	(36,007)	(26,275)
Investing activities
Purchases of short-term investments	(4,453)	(30,231)
Sales and maturities of short-term investments	37,929	47,522
Purchases of property and equipment	(124)	(266)
Acquisition of intangibles	(16)	(34)
Net cash provided by investing activities	33,336	16,991
Financing activities
Proceeds from borrowing under term loan, net	0	19,819
Proceeds from issuance of common stock	265	363
Proceeds from exercise of common stock options	3	265
Principal payments on other long-term obligations	0	(83)
Net cash provided by financing activities	268	20,364
Net (decrease) increase in cash and cash equivalents	(2,403)	11,080
Cash and cash equivalents at beginning of period	14,941	15,960
Cash and cash equivalents at end of period	12,538	27,040
Supplemental disclosure of cash flow information
Net changes in restricted cash	0	(794)
Interest paid	(946)	(54)
Income taxes paid	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	0	6,545
Amounts accrued for property and equipment	110	221
Amounts accrued for patent expenditures	0	7
Unpaid debt issuance costs	$ 0	$ 38