Goodwill and Other Intangibles and Mortgage Servicing Rights - Other intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 15,888		$ 16,941
Amortization during the period	1,053	$ 1,182
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	29,015		29,015
Accumulated amortization	(17,379)		(16,453)
Net book value	11,636		12,562
Additions during the period	0		0
Amortization during the period	926		3,915
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523
Accumulated amortization	(1,271)		(1,144)
Net book value	4,252		4,379
Additions during the period	0		290
Amortization during the period	$ 127		$ 474