Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (86,372)	$ (59,978)	$ (63,386)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	8,759	7,483	7,321
Non-cash interest expense	3,683	397	408
Change in fair value of derivative liability	(4,310)
Depreciation and amortization expense	2,530	2,286	1,625
(Gain)/loss on disposal of property and equipment	7		(5)
Purchase of premium on marketable securities			(3)
Amortization of premium on marketable securities			17
Changes in operating assets and liabilities:
Accounts receivable	(2,347)	25	24
Prepaid expenses and other current assets	(518)	(260)	45
Inventory	(737)	(95)	(9)
Operating lease assets	733
Accounts payable	124	(796)	932
Accrued expenses and deferred rent	1,714	1,711	2,558
Operating lease liabilities	(844)
Net cash used in operating activities	(77,578)	(49,227)	(50,473)
Cash flows from investing activities:
Purchases of property and equipment	(2,238)	(1,889)	(8,252)
Proceeds from sale of property and equipment			5
Purchases of marketable securities			(3,000)
Proceeds from maturities of marketable securities			38,200
Net cash (used in) provided by investing activities	(2,238)	(1,889)	26,953
Cash flows from financing activities:
Proceeds from issuance of notes payable		12,032	3,700
Proceeds from issuance of 2026 convertible notes, net of issuance costs	37,275
Proceeds from exercise of stock options	78	397	685
Proceeds from issuance of common stock pursuant to employee stock purchase plan	451	297	276
Proceeds from issuance of common stock offering, net	37,537	61,571	29,238
Payments of insurance costs financed by a third party			(591)
Repayment of notes payable		(5,657)	(1,300)
Net cash provided by financing activities	75,341	68,640	32,008
Net increase (decrease) in cash, cash equivalents and restricted cash	(4,475)	17,524	8,488
Cash, cash equivalents and restricted cash at beginning of period	60,676	43,152	34,664
Cash, cash equivalents and restricted cash at end of period	56,201	60,676	43,152
Supplemental disclosure of cash flow information:
Cash paid for interest	2,298	1,515	1,461
Supplemental disclosure of non-cash investing and financing activities:
Additional right of use asset and related lease liability	2,044
Additions to property and equipment included in accounts payable and accrued expenses at balance sheet dates	214	155	538
Derivative liability in connection with issuance of 2026 convertible notes	$ 16,434
Public offering costs included in accounts payable and accrued expenses at balance sheet dates		$ 42	$ 108