Commitments and contingencies - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jul. 31, 2019	May 30, 2019
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset	$ 11,663,000		$ 6,156,000
Operating lease liability	11,915,000
Rent expense	500,000	$ 400,000
Incremental borrowing rate, percent			7.95%
Licensing agreement, amount	1,600,000.0
Licensing agreement, milestone payment, amount	$ 300,000
Lease Extension for office and lab space
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset				$ 5,900,000	$ 5,400,000
Operating lease liability				$ 5,800,000	$ 5,300,000
Operating lease, renewal term	5 years
Master Facilities Operating Lease Due February 28, 2025
Operating Leased Assets [Line Items]
Restricted cash and cash equivalents			$ 600,000