STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (165,297)	$ (120,088)	$ (31,781)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	290	357	462
Stock-based compensation	8,452	7,997	4,743
Change in fair value of warrants			(11,723)
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	124
Issuance of common stock in a license agreement	87
(Increase) decrease in:
Receivables	425	(301)
Prepaid expenses and other current assets	(12,452)	(10,214)	809
Other noncurrent assets		(3)	37
Increase (decrease) in:
Accounts payable	(1,852)	4	1,582
Accrued expenses	203	1,724	827
Deferred revenue	(6,861)	53,418	(1,373)
Deferred rent	(380)	(189)	(213)
Other noncurrent liabilities			(20)
Net cash used in operating activities	(58,325)	(10)	(36,650)
Cash flows from investing activities:
Purchases of property and equipment	(551)	(412)	(193)
Net cash used in investing activities	(551)	(412)	(193)
Cash flows from financing activities:
Proceeds from exercise of stock options	714	4,568	1,386
Payments to employees for repurchase of restricted common stock	(1,500)	(518)	(544)
Proceeds from exercise of warrants			10,600
Repurchase of common stock	(2)	(34)
Proceeds from issuance of common stock, net		94,320
Net cash provided by (used in) financing activities	(788)	98,336	11,442
Net decrease in cash and cash equivalents	(59,664)	97,914	(25,401)
Cash and cash equivalents, beginning of period	140,717	42,803	68,204
Cash and cash equivalents, end of period	81,053	140,717	42,803
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:
Exercise of equity-classified warrants to common shares			692
Exercise of liability-classified warrants to common shares			$ 54
Common Stock
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	$ 7,123