Goodwill and Other Intangibles and Mortgage Servicing Rights - Other intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 12,896		$ 12,896		$ 14,769
Amortization during the period	880	$ 969	1,873	$ 2,022
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	30,715		30,715		30,715
Accumulated amortization	(21,437)		(21,437)		(19,818)
Net book value	9,278		9,278		10,897
Additions during the period			0		1,700
Amortization during the period			1,619		3,365
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523		5,523
Accumulated amortization	(1,905)		(1,905)		(1,651)
Net book value	$ 3,618		3,618		3,872
Additions during the period			0		0
Amortization during the period			$ 254		$ 507