CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (18,439)	$ (16,331)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Depreciation and amortization	3,382	2,050
Noncash lease expense	673
Stock-based compensation expense	3,657	924
Loss on debt extinguishment	1,704	4,658
Change in fair value of convertible preferred stock warrant liability	1,403	166
Change in fair value of compound derivative instrument		(574)
Accretion of noncash interest and debt reduction	156	1,137
Other	73	(4)
Changes in operating assets and liabilities
Accounts receivable	(75)	(1,160)
Inventories and other deferred costs	(1,173)	(1,878)
Prepaid expenses and other assets	(3,410)	(818)
Accounts payable	474	773
Accrued and other current liabilities	1,076	542
Contract liabilities	(9,170)	12,955
Other long-term liabilities	(805)	(71)
Net cash (used in) provided by operating activities	(20,474)	2,369
Cash flows from investing activities:
Purchase of available-for-sale debt securities	(40,262)
Purchase of property and equipment	(6,469)	(7,181)
Net cash used in investing activities	(46,731)	(7,181)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions	144,025
Payment of costs related to initial public offering	(3,950)
Proceeds from borrowings	20,000
Payment of costs related to issuance of Series C redeemable convertible preferred stock		(18)
Payments of borrowing costs	(490)
Repayments under borrowing arrangements	(25,000)	(645)
Debt extinguishment costs	(794)
Proceeds from exercise of common stock warrants	8
Proceeds from exercise of stock options	678	34
Net cash provided by (used in) financing activities	134,477	(629)
Effect of exchange rates on cash flows and cash equivalents	(3)	(1)
Net increase (decrease) in cash and cash equivalents	67,269	(5,442)
Cash and cash equivalents, beginning of period	19,744	22,617
Cash and cash equivalents, end of period	$ 87,013	$ 17,175