Significant agreements - Cancer Research UK (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Dec. 13, 2016	Dec. 31, 2019	Dec. 31, 2016	Mar. 31, 2020	Mar. 31, 2019
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	$ 2.0
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%