Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 100.3		$ 107.9
Recorded Investment With No Allowance	40.4		51.4
Recorded Investment With Allowance	36.1		31.8
Total Recorded Investment	76.5		83.2
Related Allowance	10.6		10.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	79.7	$ 101.2
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.2
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	44.0		45.6
Recorded Investment With No Allowance	17.5		20.9
Recorded Investment With Allowance	16.1		14.1
Total Recorded Investment	33.6		35.0
Related Allowance	3.4		3.9
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	9.2		10.0
Recorded Investment With No Allowance	3.4		3.4
Recorded Investment With Allowance	0.0		0.5
Total Recorded Investment	3.4		3.9
Related Allowance	0.0		0.0
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	0.8		1.8
Recorded Investment With No Allowance	0.2		1.7
Recorded Investment With Allowance	0.5		0.0
Total Recorded Investment	0.7		1.7
Related Allowance	0.1		0.0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.6		4.7
Recorded Investment With No Allowance	0.3		0.4
Recorded Investment With Allowance	3.4		3.5
Total Recorded Investment	3.7		3.9
Related Allowance	2.1		2.2
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	14.6		16.5
Recorded Investment With No Allowance	3.9		5.5
Recorded Investment With Allowance	3.9		4.0
Total Recorded Investment	7.8		9.5
Related Allowance	2.2		2.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	9.7		11.5
Recorded Investment With No Allowance	6.2		8.2
Recorded Investment With Allowance	2.1		2.0
Total Recorded Investment	8.3		10.2
Related Allowance	0.4		0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.6		3.7
Recorded Investment With No Allowance	4.4		3.6
Recorded Investment With Allowance	0.0		0.0
Total Recorded Investment	4.4		3.6
Related Allowance	0.0		0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	72.9		77.3
Recorded Investment With No Allowance	32.0		38.2
Recorded Investment With Allowance	22.1		20.1
Total Recorded Investment	54.1		58.3
Related Allowance	6.0		6.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	56.1	66.1
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.2
Total consumer loans
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	0.0	0.0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	25.4		29.5
Recorded Investment With No Allowance	6.7		12.4
Recorded Investment With Allowance	13.7		11.4
Total Recorded Investment	20.4		23.8
Related Allowance	4.5		4.4
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	22.1	32.7
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.0
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	2.0		1.1
Recorded Investment With No Allowance	1.7		0.8
Recorded Investment With Allowance	0.3		0.3
Total Recorded Investment	2.0		1.1
Related Allowance	0.1		$ 0.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	1.5	2.4
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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