Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (36,463)	$ (18,146)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	465	259
Stock-based compensation expense	3,278	280
Accrued interest on related party loan		270
Net amortization of premiums and accretion of discounts on investments	(358)
Non-cash lease expense	1,265	625
Changes in assets and liabilities:
Prepaid expenses and other current assets	3,545	(753)
Income tax receivable	35
Accounts payable	(905)	1,421
Accrued liabilities	894	(1,105)
Lease liabilities	(1,220)	(511)
Other current liabilities		(9)
Net cash used in operating activities	(29,464)	(17,669)
Cash flows from investing activities:
Purchase of property, plant and equipment	(4,229)	(1,148)
Purchases of investments	(127,302)
Maturities of investments	157,107
Net cash provided by (used in) investing activities	25,576	(1,148)
Cash flows from financing activities:
Proceeds from common stock issuance		11
Proceeds for exercise of stock options	39	164
Payment of employee taxes and exercise costs for shares withheld	(568)
Proceeds from purchases under the employee stock purchase plan	317
Payment of issuance costs and deferred offering costs		(122)
Net cash provided by (used in) financing activities	(212)	59,602
Net increase (decrease) in cash, cash equivalents and restricted cash	(4,100)	40,785
Cash, cash equivalents and restricted cash, beginning of period	35,891	1,987
Cash, cash equivalents and restricted cash, end of period	31,791	42,772
Cash, cash equivalents and restricted cash, end of period
Cash and cash equivalents	31,791	42,672
Restricted cash		100
Cash, cash equivalents and restricted cash, end of period	31,791	42,772
Supplemental disclosure of non-cash investing and financing activities:
Acquisition of property, plant and equipment in accounts payable and accrued liabilities	$ 3,044	334
Deferred offering costs included in accounts payable and accrued liabilities		1,879
Series C Convertible Preferred Stock
Supplemental disclosure of non-cash investing and financing activities:
Issuance costs in accounts payable and accrued liabilities		218
Haldor Topsøe Holding A/S and Holdco
Cash flows from financing activities:
Proceeds from related party capital contribution		2,549
Proceeds from loan from related party		15,000
Supplemental disclosure of non-cash investing and financing activities:
Settlement of related party loan through issuance of Series C convertible preferred stock		20,000
Haldor Topsøe Holding A/S and Holdco | Series C Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock		10,000
New Investors | Series C Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock		$ 32,000