Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	$ 76.4		$ 76.4		$ 83.0
Recorded Investment With No Allowance	27.3		27.3		36.0
Recorded Investment With Allowance	29.6		29.6		28.8
Total Recorded Investment	56.9		56.9		64.8
Related Allowance	6.2		6.2		6.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	59.3	$ 79.0	60.9	$ 82.5
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.1	0.2	0.1	0.2
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	24.9		24.9		22.2
Recorded Investment With No Allowance	11.6		11.6		8.6
Recorded Investment With Allowance	7.6		7.6		7.7
Total Recorded Investment	19.2		19.2		16.3
Related Allowance	0.9		0.9		0.7
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	6.7		6.7		10.0
Recorded Investment With No Allowance	0.4		0.4		0.4
Recorded Investment With Allowance	0.1		0.1		3.5
Total Recorded Investment	0.5		0.5		3.9
Related Allowance	0.0		0.0		0.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	1.5		1.5		1.1
Recorded Investment With No Allowance	1.3		1.3		0.6
Recorded Investment With Allowance	0.0		0.0		0.4
Total Recorded Investment	1.3		1.3		1.0
Related Allowance	0.0		0.0		0.1
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	0.6		0.6		0.7
Recorded Investment With No Allowance	0.1		0.1		0.2
Recorded Investment With Allowance	0.1		0.1		0.0
Total Recorded Investment	0.2		0.2		0.2
Related Allowance	0.0		0.0		0.0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	8.8		8.8		11.8
Recorded Investment With No Allowance	1.8		1.8		1.2
Recorded Investment With Allowance	0.2		0.2		3.9
Total Recorded Investment	2.0		2.0		5.1
Related Allowance	0.0		0.0		0.3
Residential
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	6.6		6.6		8.8
Recorded Investment With No Allowance	3.3		3.3		5.7
Recorded Investment With Allowance	2.0		2.0		2.0
Total Recorded Investment	5.3		5.3		7.7
Related Allowance	0.3		0.3		0.3
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	4.8		4.8		12.9
Recorded Investment With No Allowance	2.7		2.7		12.5
Recorded Investment With Allowance	1.7		1.7		0.2
Total Recorded Investment	4.4		4.4		12.7
Related Allowance	0.1		0.1		0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	45.1		45.1		55.7
Recorded Investment With No Allowance	19.4		19.4		28.0
Recorded Investment With Allowance	11.5		11.5		13.8
Total Recorded Investment	30.9		30.9		41.8
Related Allowance	1.3		1.3		1.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	33.9	56.4	36.4	58.6
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.1	0.0	0.1
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	24.3		24.3		24.1
Recorded Investment With No Allowance	6.2		6.2		5.5
Recorded Investment With Allowance	13.1		13.1		14.4
Total Recorded Investment	19.3		19.3		19.9
Related Allowance	4.6		4.6		5.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	19.6	20.7	19.6	22.5
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.1	0.1	0.1	0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	7.0		7.0		3.2
Recorded Investment With No Allowance	1.7		1.7		2.5
Recorded Investment With Allowance	5.0		5.0		0.6
Total Recorded Investment	6.7		6.7		3.1
Related Allowance	0.3		0.3		$ 0.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	5.8	1.9	4.9	1.4
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0	$ 0.0	$ 0.0