Stock incentive plan - Employee Stock Purchase Plan (Details) - shares	Jun. 05, 2018	Dec. 31, 2019	Dec. 31, 2018
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for issuance		5,830,000	4,154,000
Employee Stock Purchase Plan
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for issuance	314,697
Maximum percentage increase in shares available for issuance under the ESPP	1.00%