Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 107.9	$ 132.8	$ 120.4
Recorded Investment With No Allowance	51.4	57.9	56.3
Recorded Investment With Allowance	31.8	46.8	35.7
Total Recorded Investment	83.2	104.7	92.0
Related Allowance	10.8	14.0	11.6
Average Recorded Investment	94.0	101.1	80.4
Income Recognized	0.5	0.6	1.0
Impaired Loans, interest lost on nonaccrual loans	3.5	4.2	3.9
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	45.6	57.0	58.2
Recorded Investment With No Allowance	20.9	24.4	27.9
Recorded Investment With Allowance	14.1	21.4	17.6
Total Recorded Investment	35.0	45.8	45.5
Related Allowance	3.9	2.8	3.4
Average Recorded Investment	40.4	46.9	39.2
Income Recognized	0.3	0.2	0.8
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	10.0	12.1	15.5
Recorded Investment With No Allowance	3.4	4.3	7.2
Recorded Investment With Allowance	0.5	1.8	0.8
Total Recorded Investment	3.9	6.1	8.0
Related Allowance	0.0	0.8	0.3
Average Recorded Investment	5.0	8.0	8.3
Income Recognized	0.0	0.1	0.1
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.8	1.6	1.0
Recorded Investment With No Allowance	1.7	0.2	0.3
Recorded Investment With Allowance	0.0	0.6	0.0
Total Recorded Investment	1.7	0.8	0.3
Related Allowance	0.0	0.0	0.0
Average Recorded Investment	1.3	0.8	0.3
Income Recognized	0.0	0.0	0.0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.7	4.8	1.3
Recorded Investment With No Allowance	0.4	3.9	0.3
Recorded Investment With Allowance	3.5	0.7	0.7
Total Recorded Investment	3.9	4.6	1.0
Related Allowance	2.2	0.7	0.7
Average Recorded Investment	4.2	2.9	1.9
Income Recognized	0.0	0.0	0.0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	16.5	18.5	17.8
Recorded Investment With No Allowance	5.5	8.4	7.8
Recorded Investment With Allowance	4.0	3.1	1.5
Total Recorded Investment	9.5	11.5	9.3
Related Allowance	2.2	1.5	1.0
Average Recorded Investment	10.5	11.7	10.5
Income Recognized	0.0	0.1	0.1
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	11.5	8.2	7.1
Recorded Investment With No Allowance	8.2	4.1	3.6
Recorded Investment With Allowance	2.0	2.5	2.3
Total Recorded Investment	10.2	6.6	5.9
Related Allowance	0.1	0.3	0.4
Average Recorded Investment	8.4	6.2	4.1
Income Recognized	0.0	0.0	0.0
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	3.7	5.1	6.4
Recorded Investment With No Allowance	3.6	4.5	5.6
Recorded Investment With Allowance	0.0	0.2	0.2
Total Recorded Investment	3.6	4.7	5.8
Related Allowance	0.0	0.0	0.0
Average Recorded Investment	4.2	4.4	7.2
Income Recognized	0.0	0.0	0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	77.3	88.8	89.5
Recorded Investment With No Allowance	38.2	41.4	44.9
Recorded Investment With Allowance	20.1	27.2	21.6
Total Recorded Investment	58.3	68.6	66.5
Related Allowance	6.2	4.6	4.8
Average Recorded Investment	63.5	69.2	61.0
Income Recognized	0.3	0.3	0.9
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	29.5	40.3	29.6
Recorded Investment With No Allowance	12.4	13.2	10.8
Recorded Investment With Allowance	11.4	19.2	13.7
Total Recorded Investment	23.8	32.4	24.5
Related Allowance	4.4	9.3	6.5
Average Recorded Investment	28.1	30.1	18.5
Income Recognized	0.2	0.3	0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.1	3.7	1.3
Recorded Investment With No Allowance	0.8	3.3	0.6
Recorded Investment With Allowance	0.3	0.4	0.4
Total Recorded Investment	1.1	3.7	1.0
Related Allowance	0.2	0.1	0.3
Average Recorded Investment	2.4	1.8	0.9
Income Recognized	$ 0.0	$ 0.0	$ 0.0