Significant agreements - Cancer Research UK (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2016	Sep. 30, 2021	Dec. 31, 2020	Dec. 13, 2016
Research and development arrangement obligation to repay other parties
Contingent future milestones payments under research and development arrangement	$ 60.3				$ 50.9
Other long term liabilities
Research and development arrangement obligation to repay other parties
Liability from research and development			$ 3.0	$ 2.6
BT1718 | Minimum
Research and development arrangement obligation to repay other parties
Tiered royalties (percentage)		70.00%
BT1718 | Maximum
Research and development arrangement obligation to repay other parties
Tiered royalties (percentage)		90.00%
BT7401 | Minimum
Research and development arrangement obligation to repay other parties
Tiered royalties (percentage)	55.00%
BT7401 | Maximum
Research and development arrangement obligation to repay other parties
Tiered royalties (percentage)	80.00%