Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 80.0		$ 83.0
Recorded Investment With No Allowance	26.1		36.0
Recorded Investment With Allowance	35.1		28.8
Total Recorded Investment	61.2		64.8
Related Allowance	7.1		6.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	63.1	$ 79.7
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	27.0		22.2
Recorded Investment With No Allowance	10.7		8.6
Recorded Investment With Allowance	10.6		7.7
Total Recorded Investment	21.3		16.3
Related Allowance	0.7		0.7
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	10.0		10.0
Recorded Investment With No Allowance	0.4		0.4
Recorded Investment With Allowance	3.4		3.5
Total Recorded Investment	3.8		3.9
Related Allowance	0.3		0.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.5		1.1
Recorded Investment With No Allowance	0.9		0.6
Recorded Investment With Allowance	0.4		0.4
Total Recorded Investment	1.3		1.0
Related Allowance	0.0		0.1
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	0.6		0.7
Recorded Investment With No Allowance	0.1		0.2
Recorded Investment With Allowance	0.0		0.0
Total Recorded Investment	0.1		0.2
Related Allowance	0.0		0.0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	12.1		11.8
Recorded Investment With No Allowance	1.4		1.2
Recorded Investment With Allowance	3.8		3.9
Total Recorded Investment	5.2		5.1
Related Allowance	0.3		0.3
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	8.9		8.8
Recorded Investment With No Allowance	4.2		5.7
Recorded Investment With Allowance	3.3		2.0
Total Recorded Investment	7.5		7.7
Related Allowance	0.5		0.3
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	2.7		12.9
Recorded Investment With No Allowance	2.4		12.5
Recorded Investment With Allowance	0.2		0.2
Total Recorded Investment	2.6		12.7
Related Allowance	0.0		0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	50.7		55.7
Recorded Investment With No Allowance	18.7		28.0
Recorded Investment With Allowance	17.9		13.8
Total Recorded Investment	36.6		41.8
Related Allowance	1.5		1.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	39.3	56.1
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	24.3		24.1
Recorded Investment With No Allowance	5.8		5.5
Recorded Investment With Allowance	14.0		14.4
Total Recorded Investment	19.8		19.9
Related Allowance	5.1		5.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	19.9	22.1
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	5.0		3.2
Recorded Investment With No Allowance	1.6		2.5
Recorded Investment With Allowance	3.2		0.6
Total Recorded Investment	4.8		3.1
Related Allowance	0.5		$ 0.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	3.9	1.5
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0