License agreements - Zai Lab (Details) - Zai Lab Ltd - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 31, 2019	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Upfront and immediate milestone payment to be received under license agreement	$ 17.5
Research and development expenses reimbursed		$ 0.0	$ 0.2
Reimbursable costs included in accounts receivable		$ 0.2	$ 0.2	$ 0.5
Development and Regulatory Milestones | Maximum
Upfront payment under license agreement	22.5
Sales and Commercial Milestones | Maximum
Upfront payment under license agreement	$ 37.5