STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (53,117)	$ (54,323)	$ (165,297)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	575	369	290
Stock-based compensation	7,534	8,454	8,452
Preferred stock issued in exchange for 2016 ECP amendment			118,936
Change in fair value of derivative liabilities	(158)	1,295	124
Issuance of common stock in a license agreement			87
(Increase) decrease in:
Receivables	(1,845)	2	425
Prepaid expenses and other current assets	(1,263)	3,992	(12,452)
Other noncurrent assets	3,942	(12,991)
Increase (decrease) in:
Accounts payable	(3,709)	4,261	(1,852)
Accrued expenses	(1,145)	800	203
Deferred revenue	(146)	(6,389)	(6,861)
Deferred rent	(125)	(139)	(380)
Net cash used in operating activities	(49,457)	(54,669)	(58,325)
Cash flows from investing activities:
Purchases of property and equipment	(459)	(737)	(551)
Net cash used in investing activities	(459)	(737)	(551)
Cash flows from financing activities:
Proceeds from exercise of stock options	240	88	714
Issuance of restricted common stock		(2,059)	(1,500)
Repurchase of common stock	(1,622)		(2)
Proceeds from issuance of common stock, net		47,270
Proceeds from underwritten financing	47,101
Cash paid for settlement of related party relationship	(5,408)
Net cash provided by (used in) financing activities	40,311	45,299	(788)
Net decrease in cash and cash equivalents, and restricted cash	(9,605)	(10,107)	(59,664)
Cash and cash equivalents, and restricted cash, beginning of period	71,334	81,441	140,717
Cash and cash equivalents, and restricted cash, end of period	61,729	71,334	81,441
Supplementary disclosure of cash flow information:
Cash paid for interest	0	0	0
Cash paid for income taxes	0	0	0
Supplementary disclosure of noncash investing and financing activities:
Noncash portion of related party relationship settlement	212,769
Series 1 Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Payment of dividends in preferred stock	$ 16,998	$ 18,938	7,123
Series 1 preferred stock issued as consideration for a license agreement			$ 119,045