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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 13,062		$ 13,062		$ 14,769
Amortization during the period	834	$ 914	2,707	$ 2,936
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	31,715		31,715		30,715
Accumulated amortization	(22,145)		(22,145)		(19,818)
Net book value	9,570		9,570		10,897
Additions during the period			1,000		1,700
Amortization during the period			2,327		3,365
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523		5,523
Accumulated amortization	(2,031)		(2,031)		(1,651)
Net book value	$ 3,492		3,492		3,872
Additions during the period			0		0
Amortization during the period			$ 380		$ 507