Stockholder's equity (deficit) - Restricted stock activity narrative (Details) - shares	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Number of Shares
Unvested restricted common stock	421,066	80,190
Early exercise of unvested stock options	491,207
Restricted stock
Number of Shares
Unvested restricted common stock	421,066
Early exercise of unvested stock options	383,320
Restricted stock \| Founders
Number of Shares
Unvested restricted common stock	37,746

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