Segment reporting (Details)	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Research and development expense:	$ 73,020,000	$ 148,269,000	$ 173,385,000
General and administrative expense	40,813,000	59,543,000	56,963,000
Stock-based compensation	16,458,000	21,136,000	21,522,000
Restructuring expenses	0	8,713,000	0
Interest income	8,436,000	12,073,000	2,883,000
Interest expense	(3,874,000)	(4,073,000)	(3,328,000)
Income tax expense	(418,000)	0	0
Segment and consolidated net loss	$ (69,192,000)	(171,670,000)	(204,212,000)
Number of reportable segments | segment	1
Reportable Segment
Segment Reporting Information [Line Items]
Collaboration revenue	$ 40,497,000	36,855,000	26,581,000
Internal research and development expense	40,339,000	63,539,000	57,074,000
General and administrative expense	33,244,000	49,450,000	46,827,000
Stock-based compensation	16,458,000	21,136,000	21,522,000
Restructuring expenses	0	8,713,000	0
Interest income	8,436,000	12,073,000	2,883,000
Interest expense	(3,874,000)	(4,073,000)	(3,328,000)
Income tax expense	(418,000)	0	0
Segment and consolidated net loss	(69,192,000)	(171,670,000)	(204,212,000)
Emi-Le External Costs | Reportable Segment
Segment Reporting Information [Line Items]
Research and development expense:	17,642,000	14,098,000	15,032,000
XMT-2056 External Costs | Reportable Segment
Segment Reporting Information [Line Items]
Research and development expense:	6,396,000	5,812,000	4,981,000
UpRi External Costs | Reportable Segment
Segment Reporting Information [Line Items]
Research and development expense:	379,000	48,902,000	66,119,000
XMT-1592 External Costs | Reportable Segment
Segment Reporting Information [Line Items]
Research and development expense:	0	434,000	3,802,000
Preclinical and Discovery Costs | Reportable Segment
Segment Reporting Information [Line Items]
Research and development expense:	$ (625,000)	$ 4,441,000	$ 14,991,000