Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 92.5		$ 92.5		$ 107.9
Recorded Investment With No Allowance	38.7		38.7		51.4
Recorded Investment With Allowance	33.7		33.7		31.8
Total Recorded Investment	72.4		72.4		83.2
Related Allowance	8.2		8.2		10.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	77.1	$ 93.8	77.7	$ 97.5
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.1	0.2	0.5
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	29.1		29.1		45.6
Recorded Investment With No Allowance	12.5		12.5		20.9
Recorded Investment With Allowance	8.7		8.7		14.1
Total Recorded Investment	21.2		21.2		35.0
Related Allowance	1.7		1.7		3.9
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	10.4		10.4		10.0
Recorded Investment With No Allowance	0.7		0.7		3.4
Recorded Investment With Allowance	3.5		3.5		0.5
Total Recorded Investment	4.2		4.2		3.9
Related Allowance	0.2		0.2		0.0
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	0.7		0.7		1.8
Recorded Investment With No Allowance	0.2		0.2		1.7
Recorded Investment With Allowance	0.4		0.4		0.0
Total Recorded Investment	0.6		0.6		1.7
Related Allowance	0.1		0.1		0.0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.4		4.4		4.7
Recorded Investment With No Allowance	0.2		0.2		0.4
Recorded Investment With Allowance	3.3		3.3		3.5
Total Recorded Investment	3.5		3.5		3.9
Related Allowance	1.2		1.2		2.2
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	15.5		15.5		16.5
Recorded Investment With No Allowance	1.1		1.1		5.5
Recorded Investment With Allowance	7.2		7.2		4.0
Total Recorded Investment	8.3		8.3		9.5
Related Allowance	1.5		1.5		2.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	8.8		8.8		11.5
Recorded Investment With No Allowance	5.6		5.6		8.2
Recorded Investment With Allowance	2.1		2.1		2.0
Total Recorded Investment	7.7		7.7		10.2
Related Allowance	0.4		0.4		0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	12.8		12.8		3.7
Recorded Investment With No Allowance	12.6		12.6		3.6
Recorded Investment With Allowance	0.0		0.0		0.0
Total Recorded Investment	12.6		12.6		3.6
Related Allowance	0.0		0.0		0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	66.2		66.2		77.3
Recorded Investment With No Allowance	31.8		31.8		38.2
Recorded Investment With Allowance	18.0		18.0		20.1
Total Recorded Investment	49.8		49.8		58.3
Related Allowance	3.6		3.6		6.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	54.5	59.9	54.1	63.0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.0	0.1	0.1	0.4
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	24.1		24.1		29.5
Recorded Investment With No Allowance	5.2		5.2		12.4
Recorded Investment With Allowance	15.2		15.2		11.4
Total Recorded Investment	20.4		20.4		23.8
Related Allowance	4.4		4.4		4.4
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	20.7	31.6	22.0	31.9
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
Unpaid Total Principal Balance	2.2		2.2		1.1
Recorded Investment With No Allowance	1.7		1.7		0.8
Recorded Investment With Allowance	0.5		0.5		0.3
Total Recorded Investment	2.2		2.2		1.1
Related Allowance	0.2		0.2		$ 0.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	1.9	2.3	1.6	2.6
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0	$ 0.0	$ 0.0