Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (57,457)	$ (61,816)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	1,942	1,589
Stock-based compensation	6,531	9,453
Amortization of premium on investments, net	271	525
Other	206	338
Change in operating assets and liabilities:
Contracts and other receivables	1,124	9,735
Prepaid materials	(360)	(1,329)
Prepaid expenses and other assets	2,405	(634)
Accounts payable	(579)	3,007
Accrued liabilities	(652)	(1,187)
Accrued compensation	(631)	(102)
Deferred revenue	(54)	(1,176)
Deferred rent and other liabilities	(418)	(178)
Net cash used in operating activities	(47,672)	(41,775)
Investing activities
Purchases of short-term investments	(48,539)	(60,716)
Sales and maturities of short-term investments	58,310	81,437
Purchases of property and equipment	(217)	(746)
Acquisition of intangibles	(16)	(48)
Net cash provided by investing activities	9,538	19,927
Financing activities
Proceeds from borrowing under term loan, net	0	19,768
Proceeds from issuance of common stock, net	43,411	641
Proceeds from exercise of common stock options	4	309
Principal payments on other long-term obligations	0	(124)
Net cash provided by financing activities	43,415	20,594
Net increase (decrease) in cash and cash equivalents	5,281	(1,254)
Cash and cash equivalents at beginning of period	14,941	15,960
Cash and cash equivalents at end of period	20,222	14,706
Supplemental disclosure of cash flow information
Net changes in restricted cash	0	(1,176)
Interest paid	(1,444)	(518)
Income taxes paid	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	0	6,653
Amounts accrued for property and equipment	0	14
Amounts accrued for patent expenditures	$ 0	$ 5