License, collaboration and commercial agreements - Chugai Pharmaceutical Co., Ltd (Chugai) (Details) - Chugai Pharmaceutical Co., Ltd. - License and Collaboration Agreement - USD ($) $ in Millions		1 Months Ended	3 Months Ended
	Jan. 07, 2020	Feb. 29, 2020	Mar. 31, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
One-time cash payment		$ 3.0
Agreement obligations expire from first commercial sale of product (in years)	12 years
Number of days written prior notice required for other party to terminate agreement	180 days
Number of days written notice required to terminate agreement if fails to cure breach	120 days
Research and development expense.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
One-time cash payment			$ 3.0