Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (88,045)	$ (28,208)	$ (64,257)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,010	1,245	1,257
Loss on disposal of fixed assets	0	0	20
Net amortization of premiums and discounts on investments	(86)	(222)	(296)
Stock-based compensation	7,172	4,872	3,884
Change in deferred rent	0	0	110
Other non-cash items	148	103	0
Changes in operating assets and liabilities:
Accounts receivable	0	459	325
Prepaid expenses and other current assets	(1,950)	2,179	(1,690)
Other assets	(700)	0	(1,132)
Accounts payable	942	(3,110)	7,375
Accrued expenses	7,280	(3,569)	5,431
Operating lease assets	1,642	1,771	0
Operating lease liabilities	(1,281)	(1,883)	0
Deferred revenue	(828)	(41,381)	(6,243)
Net cash used in operating activities	(74,696)	(67,744)	(55,216)
Cash flows from investing activities
Maturities of marketable securities	37,500	27,000	88,565
Purchase of marketable securities	0	(53,688)	0
Purchase of property and equipment	(473)	(605)	(1,370)
Net cash provided by (used in) investing activities	37,027	(27,293)	87,195
Cash flows from financing activities
Proceeds from exercise of stock options	3,138	175	918
Proceeds from purchases of common stock under ESPP	561	489	146
Proceeds from issuance of debt, net of issuance costs	(197)	4,965	0
Payments under capital lease obligations	(116)	(87)	0
Net cash provided by financing activities	230,412	97,704	1,064
Increase in cash, cash equivalents and restricted cash	192,743	2,667	33,043
Cash, cash equivalents and restricted cash, beginning of period	62,672	60,005	26,962
Cash, cash equivalents and restricted cash, end of period	255,415	62,672	60,005
Supplemental disclosures of non-cash activities:
Fair value of common stock retired in exchange for issuance of common stock warrant	0	8,986	0
Purchases of property and equipment in accounts payable and accrued expenses	102	0	317
Debt financing costs in accrued expenses	0	180	0
Cash paid for interest	234	132	0
Right-of-use assets obtained in exchange for operating lease liabilities	9,980	4,369	0
Right-of-use assets obtained in exchange for financing lease liabilities	0	429	0
Adjustment to accumulated deficit and deferred revenue upon adoption of Topic 606	0	0	2,031
Public Stock Offering
Cash flows from financing activities
Net proceeds from public offering of common stock	163,990	92,162	0
At market equity offering program (2018 ATM)
Cash flows from financing activities
Net proceeds from public offering of common stock	$ 63,036	$ 0	$ 0