Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans	$ 7,708.4	$ 7,567.7
Mortgage loans held for sale	50.3	46.6
Total loans	7,758.7	7,614.3
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	10.0	3.7
Current Loans	2,822.2	2,792.4
Non-accrual Loans	21.1	26.8
Total Loans	2,853.3	2,822.9
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.0	7.9
Current Loans	348.9	337.8
Non-accrual Loans	4.7	3.0
Total Loans	355.6	348.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	4.3	2.1
Current Loans	223.0	236.4
Non-accrual Loans	0.5	1.7
Total Loans	227.8	240.2
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Current Loans	130.7	115.6
Non-accrual Loans	3.6	3.8
Total Loans	134.3	119.4
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.3	10.0
Current Loans	702.6	689.8
Non-accrual Loans	8.8	8.5
Total Loans	717.7	708.3
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	9.7	15.1
Current Loans	1,470.3	1,464.1
Non-accrual Loans	6.2	8.2
Total Loans	1,486.2	1,487.4
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.5	0.5
Current Loans	162.1	154.3
Non-accrual Loans	13.1	3.4
Total Loans	175.7	158.2
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	26.5	29.3
Current Loans	5,157.2	5,100.6
Non-accrual Loans	49.2	46.9
Total Loans	5,232.9	5,176.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.5	10.3
Current Loans	787.2	772.6
Non-accrual Loans	1.7	1.8
Total Loans	795.4	784.7
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.6	2.2
Current Loans	180.5	172.6
Non-accrual Loans	0.4	0.3
Total Loans	182.5	175.1
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.7	2.2
Current Loans	75.6	72.4
Non-accrual Loans	0.0	0.0
Total Loans	77.3	74.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	9.8	14.7
Current Loans	1,043.3	1,017.6
Non-accrual Loans	2.1	2.1
Total Loans	1,055.2	1,034.4
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.8	6.3
Current Loans	1,243.0	1,189.5
Non-accrual Loans	16.2	19.6
Total Loans	1,266.0	1,215.4
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.3	1.9
Current Loans	146.1	133.5
Non-accrual Loans	1.8	0.8
Total Loans	151.2	136.2
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Current Loans	3.1	4.9
Non-accrual Loans	0.0	0.0
Total Loans	3.1	4.9
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	46.4	52.2
Current Loans	7,592.7	7,446.1
Non-accrual Loans	69.3	69.4
Total Loans	7,708.4	7,567.7
Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Current Loans	50.3	46.6
Non-accrual Loans	0.0	0.0
Mortgage loans held for sale	50.3	46.6
Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	46.4	52.2
Current Loans	7,643.0	7,492.7
Non-accrual Loans	69.3	69.4
Total loans	7,758.7	7,614.3
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	5.7	2.9
30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.7	7.3
30 to 59 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	4.3	2.1
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.0	9.4
30 to 59 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.0	13.3
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.2	0.3
30 to 59 Days Past Due | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	13.9	25.9
30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	5.5	7.8
30 to 59 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.9	1.6
30 to 59 Days Past Due | Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.6	0.9
30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	7.0	10.3
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.7	3.9
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.6	1.8
30 to 59 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
30 to 59 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	25.2	41.9
30 to 59 Days Past Due | Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
30 to 59 Days Past Due | Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	25.2	41.9
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	4.0	0.5
60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.1	0.3
60 to 89 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
60 to 89 Days Past Due | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.1	0.3
60 to 89 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	4.7	1.4
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.3	0.0
60 to 89 Days Past Due | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	9.1	2.2
60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.8	2.1
60 to 89 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.6	0.5
60 to 89 Days Past Due | Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.4	0.6
60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.8	3.2
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.8	1.7
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.4	0.1
60 to 89 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
60 to 89 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	15.1	7.2
60 to 89 Days Past Due | Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
60 to 89 Days Past Due | Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	15.1	7.2
Equal to Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.3	0.3
Equal to Greater than 90 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.2	0.3
Equal to Greater than 90 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Equal to Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Equal to Greater than 90 Days Past Due | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.2	0.3
Equal to Greater than 90 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.0	0.4
Equal to Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.2
Equal to Greater than 90 Days Past Due | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.5	1.2
Equal to Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.2	0.4
Equal to Greater than 90 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.1	0.1
Equal to Greater than 90 Days Past Due | Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.7	0.7
Equal to Greater than 90 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.0	1.2
Equal to Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.3	0.7
Equal to Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.3	0.0
Equal to Greater than 90 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Equal to Greater than 90 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.1	3.1
Equal to Greater than 90 Days Past Due | Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.0
Equal to Greater than 90 Days Past Due | Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	$ 6.1	$ 3.1