CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net loss	$ (47,731)	$ (51,058)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	4,454	9,165
Issuance of common stock for services rendered	77	119
Accretion of discounts and premiums on investments		94
Depreciation and amortization expense	395	559
Loss on disposal or impairment of property and equipment	497
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,881	(1,639)
Accounts payable, accrued expenses, and other liabilities	455	(393)
Deferred revenue	(472)	(606)
Net cash used in operating activities	(40,444)	(43,759)
Cash Flows from Investing Activities:
Proceeds from maturity of available-for-sale securities		26,870
Proceeds from the sale of property and equipment	193
Purchases of property and equipment	(71)	(100)
Net cash provided by investing activities	122	26,770
Cash Flows from Financing Activities:
Proceeds from employee stock purchases	205	184
Proceeds from exercise of common stock options and warrants	10,166	304
Payments on note payable	(209)	(216)
Payments on capital leases	(7)	(9)
Net cash provided by financing activities	10,155	263
Net decrease in cash, cash equivalents and restricted cash	(30,167)	(16,726)
Cash, cash equivalents and restricted cash, beginning of period	112,940	80,978
Cash, cash equivalents and restricted cash, end of period	$ 82,773	$ 64,252