Stockholder's equity (deficit) - Restricted stock activity narrative (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Number of Shares
Unvested restricted common stock	93,532	155,149
Founders
Number of Shares
Unvested restricted common stock	84,082
Restricted stock \| Founders
Number of Shares
Unvested restricted common stock	9,450

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant