Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	$ 87.4		$ 87.4		$ 83.0
Recorded Investment With No Allowance	38.1		38.1		36.0
Recorded Investment With Allowance	29.0		29.0		28.8
Total Recorded Investment	67.1		67.1		64.8
Related Allowance	4.1		4.1		6.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	62.0	$ 77.1	65.8	$ 77.7
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.0	0.1	0.3
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	28.4		28.4		22.2
Recorded Investment With No Allowance	13.2		13.2		8.6
Recorded Investment With Allowance	9.7		9.7		7.7
Total Recorded Investment	22.9		22.9		16.3
Related Allowance	0.7		0.7		0.7
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	6.8		6.8		10.0
Recorded Investment With No Allowance	0.4		0.4		0.4
Recorded Investment With Allowance	0.2		0.2		3.5
Total Recorded Investment	0.6		0.6		3.9
Related Allowance	0.0		0.0		0.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	0.6		0.6		1.1
Recorded Investment With No Allowance	0.4		0.4		0.6
Recorded Investment With Allowance	0.0		0.0		0.4
Total Recorded Investment	0.4		0.4		1.0
Related Allowance	0.0		0.0		0.1
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	1.0		1.0		0.7
Recorded Investment With No Allowance	0.5		0.5		0.2
Recorded Investment With Allowance	0.1		0.1		0.0
Total Recorded Investment	0.6		0.6		0.2
Related Allowance	0.0		0.0		0.0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	8.4		8.4		11.8
Recorded Investment With No Allowance	1.3		1.3		1.2
Recorded Investment With Allowance	0.3		0.3		3.9
Total Recorded Investment	1.6		1.6		5.1
Related Allowance	0.0		0.0		0.3
Residential
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	6.3		6.3		8.8
Recorded Investment With No Allowance	3.1		3.1		5.7
Recorded Investment With Allowance	2.0		2.0		2.0
Total Recorded Investment	5.1		5.1		7.7
Related Allowance	0.2		0.2		0.3
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	7.6		7.6		12.9
Recorded Investment With No Allowance	5.1		5.1		12.5
Recorded Investment With Allowance	2.1		2.1		0.2
Total Recorded Investment	7.2		7.2		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	50.7		50.7		55.7
Recorded Investment With No Allowance	22.7		22.7		28.0
Recorded Investment With Allowance	14.1		14.1		13.8
Total Recorded Investment	36.8		36.8		41.8
Related Allowance	1.0		1.0		1.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	33.9	54.5	39.2	54.1
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.0	0.0	0.2
Commercial
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	30.9		30.9		24.1
Recorded Investment With No Allowance	12.6		12.6		5.5
Recorded Investment With Allowance	12.4		12.4		14.4
Total Recorded Investment	25.0		25.0		19.9
Related Allowance	2.9		2.9		5.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	22.1	20.7	22.5	22.0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.0	0.1	0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid Total Principal Balance	5.8		5.8		3.2
Recorded Investment With No Allowance	2.8		2.8		2.5
Recorded Investment With Allowance	2.5		2.5		0.6
Total Recorded Investment	5.3		5.3		3.1
Related Allowance	0.2		0.2		$ 0.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	6.0	1.9	4.1	1.6
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0	$ 0.0	$ 0.0