Goodwill and Other Intangibles and Mortgage Servicing Rights - Other intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 13,776		$ 14,769
Amortization during the period	993	$ 1,053
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	30,715		30,715
Accumulated amortization	(20,684)		(19,818)
Net book value	10,031		10,897
Additions during the period	0		1,700
Amortization during the period	866		3,365
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523
Accumulated amortization	(1,778)		(1,651)
Net book value	3,745		3,872
Additions during the period	0		0
Amortization during the period	$ 127		$ 507