Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (24,977)	$ (50,299)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	594	592
Stock-based compensation	7,151	7,217
Amortization of discount or premium on short-term investments	(169)	307
Deferred rent	54	1,488
Changes in operating assets and liabilities:
Accounts receivable, net	93	15
Inventory	0	244
Prepaid and other assets	(64)	930
Accounts payable	(184)	0
Accrued expenses	(1,237)	(4,258)
Accrued compensation	(1,364)	(1,501)
Net cash used in operating activities	(20,103)	(45,265)
Cash flows from investing activities:
Purchases of short-term investments	(54,213)	(73,662)
Maturities of short-term investments	75,000	117,299
Purchases of property and equipment	(343)	(681)
Net cash provided by investing activities	20,444	42,956
Cash flows from financing activities:
Proceeds from exercise of stock options	32	58
Proceeds from issuance of common stock	197	427
Net cash provided by financing activities	229	485
Net change in cash, cash equivalents and restricted cash	570	(1,824)
Cash, cash equivalents and restricted cash at beginning of period	19,614	24,853
Cash, cash equivalents and restricted cash at end of period	20,184	23,029
Cash and cash equivalents at end of period	19,024	22,332
Restricted cash at end of period	1,160	697
Supplemental disclosure of non-cash investing activities:
Purchase of property and equipment in accounts payable and accrued expenses	$ 166	$ 36