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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Net book value	$ 14,005		$ 14,005		$ 16,941
Amortization during the period	914	$ 1,054	2,936	$ 3,336
Core deposit intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	29,015		29,015		29,015
Accumulated amortization	(19,009)		(19,009)		(16,453)
Net book value	10,006		10,006		12,562
Additions during the period			0		0
Amortization during the period			2,556		3,915
Customer list intangibles:
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,523		5,523		5,523
Accumulated amortization	(1,524)		(1,524)		(1,144)
Net book value	$ 3,999		3,999		4,379
Additions during the period			0		290
Amortization during the period			$ 380		$ 474