Summary of significant accounting policies - Nature of operations and Basis of presentation (Details) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2022 USD ($) Office $ / shares	Dec. 31, 2021 $ / shares	Dec. 31, 2020 $ / shares
Summary of significant accounting policies
Number of full-service community banking offices \| Office	28
Reclassification of prior period items amounts effect on net income \| $	$ 0
Dividends declared \| $ / shares	$ 1.58	$ 1.50	$ 1.44

Aggregate dividends declared during the period for each share of common stock outstanding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI https://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration