STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (54,323)	$ (165,297)	$ (120,088)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	369	290	357
Stock-based compensation	8,454	8,452	7,997
Common stock issued in exchange for license agreement			67,285
Preferred stock issued in exchange for 2016 ECP amendment		118,936
Change in fair value of derivative liabilities	1,295	124
Issuance of common stock in a license agreement		87
(Increase) decrease in:
Receivables	2	425	(301)
Prepaid expenses and other current assets	3,992	(12,452)	(10,214)
Other noncurrent assets	(12,991)		(3)
Increase (decrease) in:
Accounts payable	4,261	(1,852)	4
Accrued expenses	800	203	1,724
Deferred revenue	(6,389)	(6,861)	53,418
Deferred rent	(139)	(380)	(189)
Net cash used in operating activities	(54,669)	(58,325)	(10)
Cash flows from investing activities:
Purchases of property and equipment	(737)	(551)	(412)
Net cash used in investing activities	(737)	(551)	(412)
Cash flows from financing activities:
Proceeds from exercise of stock options	88	714	4,568
Issuance of restricted common stock	(2,059)	(1,500)	(518)
Repurchase of common stock		(2)	(34)
Proceeds from issuance of common stock, net	47,270		94,320
Net cash provided by (used in) financing activities	45,299	(788)	98,336
Net decrease in cash and cash equivalents	(10,107)	(59,664)	97,914
Cash and cash equivalents, beginning of period	81,053	140,717	42,803
Cash and cash equivalents, end of period	70,946	81,053	140,717
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:
Issuance of stock			$ 67,285
Series 1 Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Issuance of stock		119,045
Payment of dividends in preferred stock	$ 18,938	$ 7,123