Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net loss	$ (30,572)	$ (8,075)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	156	32
Stock-based compensation	2,023	722
Amortization of debt discount and issuance costs	73	194
Loss on sale of equipment	1	0
Changes in operating assets and liabilities:
Accounts receivable	(2,049)	0
Grants and other receivables	(225)	141
Prepaid expenses and other assets	(118)	1,592
Inventory	(1,543)	0
Accounts payable	3,723	(2,417)
Accrued expenses and other liabilities	5,533	281
Cash used in operating activities	(22,998)	(7,530)
Investing activities:
Purchases of property and equipment	(382)	(90)
Cash used in investing activities	(382)	(90)
Financing activities:
Proceeds from the sale of Series D preferred stock	0	35,000
Cash paid for financing costs	(1,823)	(185)
Proceeds from the exercise of stock options	130	0
Cash (used in) provided by financing activities	(1,693)	34,815
Effects of exchange rate changes on cash and cash equivalents	(1)	0
Net (decrease) increase in cash, cash equivalents and restricted cash	(25,074)	27,195
Cash, cash equivalents and restricted cash at beginning of period	234,875	36,847
Cash, cash equivalents and restricted cash at end of period	209,801	64,042
Supplemental disclosure of noncash financing activities:
Deemed dividend	0	3,067
Accretion to redemption value	0	528
Fixed asset purchases within accounts payable and accrued expenses	303	0
Financing costs within accounts payable and accrued expenses	671	0
Conversion of convertible notes payable and accrued interest into Series C-2 preferred stock	$ 0	$ 19,527