Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (78,185)	$ (41,912)	$ (21,313)
Adjustments to reconcile net loss to net cash used for operating activities:
Share-based compensation expense	14,349	11,551	8,992
Third party share-based compensation expense	197	1,521	63
Issuance of common stock for services	0	0	25
Depreciation expense	730	347	17
Loss on disposal	75	16	0
Changes in operating assets and liabilities:
Restricted cash	37	(200)	0
Prepaid clinical expenses	156	1,305	(1,528)
Prepaid expenses and other current assets	(820)	(481)	(94)
Other assets	(149)	(70)	0
Accounts payable	3,600	2,322	(104)
Accrued clinical and other expenses	351	(248)	739
Accrued payroll and related expenses	987	666	324
Net cash used for operating activities	(58,672)	(25,183)	(12,879)
Investing activities
Purchase of property and equipment	(2,218)	(1,816)	(258)
Net cash used for investing activities	(2,218)	(1,816)	(258)
Financing activities
Net proceeds from the issuance of common stock	0	104,596	53,063
Net proceeds from the exercise of stock options for common stock	149	315	0
Proceeds from the issuance of Series F Convertible Preferred Stock	0	0	0
Net cash provided by financing activities	149	104,911	53,063
Net (decrease) increase in cash and cash equivalents	(60,741)	77,912	39,926
Cash and cash equivalents at beginning of period	126,467	48,555	8,629
Cash, cash equivalents at end of period	65,726	126,467	48,555
Series C-1 Convertible Preferred Stock [Member]
Non-cash investing and financing activity:
Conversion of temporary equity into common stock	0	11	0
Series F Convertible Preferred Stock [Member]
Non-cash investing and financing activity:
Conversion of temporary equity into common stock	0	61	452
Series C-1 and D-1 Convertible Preferred Stock [Member]
Non-cash investing and financing activity:
Conversion of temporary equity into common stock	$ 0	$ 0	$ 3,099