Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities:
Net loss	$ (56,251)	$ (55,154)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	592	173
Stock-based compensation	5,117	4,123
Amortization of debt discount and issuance costs	234	180
Changes in operating assets and liabilities:
Accounts receivable	(7,001)	(3,671)
Grants and other receivables	(48)	(165)
Prepaid expenses and other assets	1,096	(859)
Inventory	2,803	(2,723)
Accounts payable	(4,253)	2,917
Accrued expenses and other liabilities	129	7,686
Cash used in operating activities	(57,582)	(47,493)
Investing activities:
Purchases of property and equipment	(423)	(881)
Cash used in investing activities	(423)	(881)
Financing activities:
Proceeds from the sale of common stock	0	63,969
Cash paid for financing costs	0	(6,349)
Proceeds from issuance of common stock under employee stock purchase plan	612	0
Proceeds from the exercise of stock options	369	857
Cash provided by financing activities	981	58,477
Effects of exchange rate changes on cash and cash equivalents	(9)	25
Net (decrease) increase in cash, cash equivalents and restricted cash	(57,033)	10,128
Cash, cash equivalents and restricted cash at beginning of period	201,011	234,875
Cash, cash equivalents and restricted cash at end of period	143,978	245,003
Supplemental disclosure of noncash activities:
Fixed asset purchases within accounts payable and accrued expenses	35	233
Fixed asset additions acquired though tenant allowance	0	267
Financing costs within accounts payable and accrued expenses	0	188
Recognition of initial right-of-use assets	3,724	0
Recognition of initial lease liabilities	$ 4,201	$ 0