CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operating activities:
Net loss	$ (124,989)	$ (196,743)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	14,341	15,743
Net amortization of premiums and accretion of discounts on marketable securities	(6,256)	92
Amortization of operating lease right-of-use assets	1,152	242
Depreciation expense	360	14
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(292)	22,366
Other non-current assets	4	3,061
Accounts payable	7,685	10,371
Accrued expenses	(5,452)	(27,554)
Operating lease liabilities	(1,159)	222
Other current liabilities	(18)	(53)
Other non-current liabilities	700	(6)
Net cash used in operating activities	(113,924)	(172,583)
Cash flows from investing activities:
Purchase of marketable securities	(91,202)	(140,360)
Maturities of marketable securities	294,583	49,000
Purchases of property and equipment	(615)	(494)
Net cash provided by (used in) investing activities	202,766	(91,854)
Cash flows from financing activities:
Proceeds from exercises of stock options	709	220
Proceeds from issuance of common stock under the employee stock purchase plan	196	149
Payments for repurchases of unvested restricted common stock	(1)	(4)
Net cash provided by financing activities	904	365
Net increase (decrease) in cash and cash equivalents	89,746	(264,072)
Cash and cash equivalents at beginning of period	92,076	542,224
Cash and cash equivalents at end of period	181,822	278,152
Supplemental disclosure of cash flow information
Operating lease right-of-use asset recognized upon adoption of ASC 842	0	1,728
Purchases of property and equipment in accounts payable and accrued expenses	$ 0	$ 867