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	$ (55,350)	$ (65,336)	$ (65,321)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	726	975	750
Amortization of premiums (accretion of discounts) on investments	(154)	(101)	233
Amortization of right-of-use assets	1,301	0	0
Loss (gain) on disposal of property and equipment	104	(81)	36
Stock‑based compensation expense	5,161	5,714	4,081
Non‑cash interest expense	402	449	390
Warrant liabilities revaluation	857	0	0
Loss on extinguishment of debt	0	0	673
Net realized losses on investments	(1)	0	0
Changes in operating assets and liabilities:
Accounts receivable	(5,000)	0	152
Prepaid expenses, deposits and other assets	3,179	(2,631)	1,225
Accounts payable	(600)	(516)	(2,327)
Deferred revenue	337	(101)	2,432
Accrued expenses and other liabilities	(2,397)	2,467	5,650
Net cash used in operating activities	(51,435)	(59,161)	(52,026)
Cash flows from investing activities
Receipts from the maturity of short-term investments	16,350	41,655	60,158
Purchases of short-term investments	(18,188)	(15,598)	(61,527)
Sale of short term investments	1,992	0	0
Purchases of property and equipment	(47)	(884)	(733)
Proceeds from the sale of property and equipment	122	99	4
Net cash provided by (used in) investing activities	229	25,272	(2,098)
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	0	0	20,957
Repayments of principal on outstanding debt	(2,800)	0	(12,934)
Net proceeds from issuance of common stock	30,942	0	10,000
Proceeds from exercise of stock options	150	501	706
Proceeds from issuance of common stock under Employee Stock Purchase Plan	28	196	180
Net cash provided by financing activities	105,041	697	66,023
Effect of exchange rate changes on cash	34	(153)	78
Net change in cash, cash equivalents, and restricted cash	53,869	(33,345)	11,977
Cash, cash equivalents, and restricted cash at beginning of period	37,682	71,027	59,050
Cash, cash equivalents, and restricted cash at end of period	91,551	37,682	71,027
Supplement cash flow information
Cash paid for interest	1,223	1,134	905
Noncash investing and financing activities
Purchase of property and equipment not yet paid	0	145	103
Issuance costs in connection with common stock, common warrants, and pre-funded warrants in accrued liabilities	4,381	0	0
Unrealized gain on marketable securities	0	16	20
At-The-Market Offering
Cash flows from financing activities
Net proceeds from issuance of common stock	1,006	0	0
Private Placement
Cash flows from financing activities
Net proceeds from issuance of common stock	5,715	0	47,114
Note Warrant
Cash flows from financing activities
Net proceeds from issuance of common stock	$ 70,000	$ 0	$ 0