Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Loans, principally due to allowance for loan losses	$ 18.0	$ 30.1
Loan discount	6.7	3.1
Investment securities, unrealized losses	5.6	6.6
Employee benefits	11.5	9.3
Non-performing loan interest	1.1	1.2
Other real estate owned write-downs and carrying costs	0.6	1.3
Tax credit carryforwards	3.7	0.0
Net operating loss carryforwards	8.7	0.0
Other	4.7	2.5
Deferred tax assets	60.6	54.1
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(6.7)	(4.7)
Deferred loan costs	(1.7)	(3.0)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.8)	(1.2)
Prepaid amounts	(0.6)	(1.5)
Government agency stock dividends	(1.6)	(2.3)
Goodwill and core deposit intangibles	(38.1)	(42.4)
Mortgage servicing rights	(5.7)	(5.5)
Other	(1.4)	(0.3)
Deferred tax liabilities	(56.6)	(60.9)
Net deferred tax assets (liabilities)	4.0
Net deferred tax assets (liabilities)		$ (6.8)
Federal
Operating Loss Carryforwards [Line Items]
Federal tax credit carryforwards	3.7
Net operating loss carryforwards	$ 20.1