STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (120,088)	$ (31,781)	$ (57,107)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	357	462	738
Stock-based compensation	7,997	4,743	3,507
Change in fair value of warrants		(11,723)	(1,185)
Common stock issued in exchange for license agreement	67,285
Loss on disposal of fixed assets			585
(Increase) decrease in:
Receivables	(301)		(87)
Prepaid expenses and other current assets	(10,214)	809	4,964
Other noncurrent assets	(3)	37	477
Increase (decrease) in:
Accounts payable	4	1,582	(1,087)
Accrued expenses	1,724	827	(10,159)
Deferred revenue	53,418	(1,373)	(800)
Deferred rent	(189)	(213)	625
Other noncurrent liabilities		(20)	20
Net cash used in operating activities	(10)	(36,650)	(59,509)
Cash flows from investing activities:
Purchases of property and equipment	(412)	(193)	(132)
Proceeds from sale of property and equipment			1
Net cash used in investing activities	(412)	(193)	(131)
Cash flows from financing activities:
Proceeds from exercise of stock options	4,568	1,386	956
Payments to employees for repurchase of restricted common stock	(518)	(544)	(498)
Proceeds from exercise of warrants		10,600	200
Repurchase of common stock	(34)
Proceeds from issuance of common stock, net	94,320		53,880
Net cash provided by financing activities	98,336	11,442	54,538
Net increase (decrease) in cash and cash equivalents	97,914	(25,401)	(5,102)
Cash and cash equivalents, beginning of period	42,803	68,204	73,306
Cash and cash equivalents, end of period	140,717	42,803	68,204
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	$ 196
Issuance of common stock in license agreement	$ 67,285
Exercise of liability-classified warrants to common shares		$ 54