Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans	$ 8,480.3	$ 7,567.7
Mortgage loans held for sale	37.7	46.6
Total loans	8,518.0	7,614.3
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	13.1	3.7
Current Loans	3,185.0	2,792.4
Non-accrual Loans	15.0	26.8
Total Loans	3,213.1	2,822.9
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.8	7.9
Current Loans	309.0	337.8
Non-accrual Loans	3.7	3.0
Total Loans	314.5	348.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.8	2.1
Current Loans	246.2	236.4
Non-accrual Loans	0.6	1.7
Total Loans	250.6	240.2
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	5.3	0.0
Current Loans	210.7	115.6
Non-accrual Loans	3.6	3.8
Total Loans	219.6	119.4
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	10.9	10.0
Current Loans	765.9	689.8
Non-accrual Loans	7.9	8.5
Total Loans	784.7	708.3
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	12.3	15.1
Current Loans	1,562.5	1,464.1
Non-accrual Loans	6.4	8.2
Total Loans	1,581.2	1,487.4
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.6	0.5
Current Loans	203.6	154.3
Non-accrual Loans	12.6	3.4
Total Loans	217.8	158.2
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	37.9	29.3
Current Loans	5,717.0	5,100.6
Non-accrual Loans	41.9	46.9
Total Loans	5,796.8	5,176.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.4	10.3
Current Loans	792.6	772.6
Non-accrual Loans	1.8	1.8
Total Loans	800.8	784.7
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.7	2.2
Current Loans	196.0	172.6
Non-accrual Loans	0.4	0.3
Total Loans	198.1	175.1
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.8	2.2
Current Loans	77.6	72.4
Non-accrual Loans	0.0	0.0
Total Loans	79.4	74.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	9.9	14.7
Current Loans	1,066.2	1,017.6
Non-accrual Loans	2.2	2.1
Total Loans	1,078.3	1,034.4
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.9	6.3
Current Loans	1,315.3	1,189.5
Non-accrual Loans	15.0	19.6
Total Loans	1,337.2	1,215.4
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.6	1.9
Current Loans	259.1	133.5
Non-accrual Loans	2.1	0.8
Total Loans	264.8	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	3.2	4.9
Non-accrual Loans	0.0	0.0
Total Loans	3.2	4.9
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	58.3	52.2
Current Loans	8,360.8	7,446.1
Non-accrual Loans	61.2	69.4
Total Loans	8,480.3	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	37.7	46.6
Non-accrual Loans	0.0	0.0
Mortgage loans held for sale	37.7	46.6
Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	58.3	52.2
Current Loans	8,398.5	7,492.7
Non-accrual Loans	61.2	69.4
Total loans	8,518.0	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	9.6	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	0.7	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	3.4	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	5.3	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	9.4	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	3.2	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	1.3	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	23.5	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	5.0	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	1.1	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	6.7	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	4.1	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	2.7	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	37.0	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	37.0	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	3.0	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	0.7	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	0.4	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	1.1	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	7.3	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	0.0	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	11.4	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	1.2	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	0.4	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	2.0	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	2.0	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	0.7	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	16.1	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	16.1	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	0.5	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	0.4	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	0.4	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	1.8	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	0.3	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	3.0	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	0.2	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	0.8	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	1.2	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	0.8	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	0.2	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	5.2	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	$ 5.2	$ 3.1