Goodwill and Other Intangibles and Mortgage Servicing Rights - Other intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 14,523		$ 14,523		$ 12,202
Amortization during the period	758	$ 834	2,400	$ 2,707
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	36,436		36,436		31,715
Accumulated amortization	(24,897)		(24,897)		(22,878)
Net book value	11,539		11,539		8,837
Additions during the period			4,721		1,000
Amortization during the period			2,019		3,060
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523		5,523
Accumulated amortization	(2,539)		(2,539)		(2,158)
Net book value	$ 2,984		2,984		3,365
Amortization during the period			$ 381		$ 507