Significant agreements - Cancer Research UK (Details) - USD ($) $ in Millions		1 Months Ended
	Dec. 13, 2016	Dec. 31, 2019	Dec. 31, 2018
Research and development arrangement obligation to repay other parties
Contingent future milestones payments under research and development arrangement	$ 50.9	$ 60.3
Other long term liabilities
Research and development arrangement obligation to repay other parties
Liability from research and development		$ 2.0	$ 0.8
Commercialization license per candidate
Research and development arrangement obligation to repay other parties
Refunding the costs and expenses incurred in case of termination (as a percent)	50.00%
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%