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	$ (54,789)	$ (6,597)	$ (45,377)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization expense	1,785	1,347	964
Stock-based compensation expense	5,991	5,217	4,709
Non-cash interest expense related to note payable	74
Net amortization of premiums and discounts on marketable securities	(764)	(377)	516
Realized losses on marketable securities			2
Loss on disposal of property and equipment	1	14	35
Non-cash operating lease cost	1,814
Changes in operating assets and liabilities:
Amounts due from related party			5,000
Prepaid expenses and other current assets	3,001	2,170	1,088
Other assets	12	(26)	(12)
Accounts payable	583	23	285
Accrued expenses and other current liabilities	(710)	(524)	2,945
Deferred rent		(52)	249
Operating lease liability	(1,778)
Deferred revenue - related party	(15,360)	(14,417)	17,174
Net cash used in operating activities	(60,140)	(13,222)	(12,422)
Cash flows from investing activities:
Purchases of property and equipment	(1,538)	(2,019)	(1,973)
Purchases of marketable securities	(118,347)	(107,257)
Proceeds from sales or maturities of marketable securities	136,850	72,692	27,891
Net cash provided by (used in) investing activities	16,965	(36,584)	25,918
Cash flows from financing activities:
Payments of initial public offering costs		(2,031)	(1,200)
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts		100,440
Proceeds from issuance of common stock to a related party		11,500
Collection of note receivable from officer			62
Proceeds from issuance of convertible note payable, net of issuance costs	7,217
Payments of debt issuance costs	(81)
Proceeds for issuance of common stock offering, net	24
Proceeds from exercise of stock options	255	467	102
Net cash provided by (used in) financing activities	7,415	110,376	(1,036)
Net increase (decrease) in cash and cash equivalents and restricted cash	(35,760)	60,570	12,460
Cash and cash equivalents and restricted cash at beginning of period	84,110	23,540	11,080
Cash and cash equivalents and restricted cash at end of period	$ 48,350	84,110	23,540
Supplemental disclosure of cash flow information:
Cash paid for income taxes		28	3,297
Supplemental disclosure of non-cash investing and financing activities:
Accretion of redeemable convertible preferred stock to redemption value		11	40
Purchases of property and equipment included in accounts payable and accrued expenses		$ 692	450
Deferred offering costs included in accrued expenses			584
Reclassification of restricted cash from non-current assets to current assets			85
Reclassification of deposit liability for restricted stock upon vesting of shares			35
Landlord incentives for construction of leasehold improvements recorded as deferred rent			$ 2,377