CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash Flows from Operating Activities:
Net loss	$ (36,126)	$ (36,528)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,127	7,542
Issuance of common stock for services rendered	45	74
Accretion of discounts and premiums on investments		92
Depreciation and amortization expense	321	368
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	69	(2,179)
Accounts payable, accrued expenses, and other liabilities	4,243	(779)
Deferred revenue	(331)	(465)
Net cash used in operating activities	(28,652)	(31,875)
Cash Flows from Investing Activities:
Proceeds from maturity of available-for-sale securities		25,695
Purchases of property and equipment	(42)	(100)
Net cash (used in) provided by investing activities	(42)	25,595
Cash Flows from Financing Activities:
Proceeds from employee stock purchases	159	115
Proceeds from exercise of common stock options and warrants	10,166	304
Payments on note payable	(209)	(142)
Payments on capital lease	(5)	(5)
Net cash provided by financing activities	10,111	272
Net decrease in cash, cash equivalents and restricted cash	(18,583)	(6,008)
Cash, cash equivalents and restricted cash, beginning of period	112,940	80,978
Cash, cash equivalents and restricted cash, end of period	$ 94,357	$ 74,970