Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (114,803)	$ (78,185)	$ (41,912)
Adjustments to reconcile net loss to net cash used for operating activities:
Share-based compensation expense	20,776	14,349	11,551
Third party share-based compensation expense	1,019	197	1,521
Depreciation expense	1,268	730	347
Loss on disposal of property and equipment	199	75	16
Changes in operating assets and liabilities:
Restricted cash, current portion	200	37	(200)
Prepaid expenses and other current assets	(1,642)	(664)	824
Other assets	219	(149)	(70)
Accounts payable	4,832	3,600	2,322
Accrued clinical and other expenses	(202)	227	(248)
Accrued payroll and related expenses	2,918	987	666
Deferred rent	335	124	0
Net cash used for operating activities	(84,881)	(58,672)	(25,183)
Investing activities
Purchase of property and equipment	(9,194)	(2,218)	(1,816)
Net cash used for investing activities	(9,194)	(2,218)	(1,816)
Financing activities
Net proceeds from the issuance of common stock	117,480	0	104,596
Net proceeds from the exercise of stock options for common stock	2,693	149	315
Restricted cash, long term	(909)	0	0
Net cash provided by financing activities	119,264	149	104,911
Net increase (decrease) in cash and cash equivalents	25,189	(60,741)	77,912
Cash and cash equivalents at beginning of period	65,726	126,467	48,555
Cash, cash equivalents at end of period	90,915	65,726	126,467
Non-cash investing and financing activity:
Capitalized landlord funded tenant improvements	13,696	0	0
Series C-1 Convertible Preferred Stock [Member]
Non-cash investing and financing activity:
Conversion of temporary equity into common stock	0	0	11
Series F Convertible Preferred Stock [Member]
Non-cash investing and financing activity:
Conversion of temporary equity into common stock	$ 0	$ 0	$ 61