Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans	$ 8,459.2	$ 8,470.4
Mortgage loans held for sale	34.0	33.3
Total loans	8,493.2	8,503.7
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	22.3	12.2
Current Loans	3,222.4	3,214.0
Non-accrual Loans	11.1	9.2
Total Loans	3,255.8	3,235.4
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.3	1.9
Current Loans	286.6	316.0
Non-accrual Loans	3.8	3.7
Total Loans	292.7	321.6
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.2	1.4
Current Loans	229.6	240.4
Non-accrual Loans	1.3	1.0
Total Loans	233.1	242.8
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.0	0.4
Current Loans	311.9	273.7
Non-accrual Loans	0.2	0.2
Total Loans	312.1	274.3
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	4.5	3.7
Current Loans	828.1	830.1
Non-accrual Loans	5.3	4.9
Total Loans	837.9	838.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	6.4	10.1
Current Loans	1,517.2	1,525.3
Non-accrual Loans	5.9	6.6
Total Loans	1,529.5	1,542.0
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	5.2	2.2
Current Loans	202.8	202.6
Non-accrual Loans	2.4	12.6
Total Loans	210.4	217.4
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	38.4	28.2
Current Loans	5,770.5	5,772.0
Non-accrual Loans	24.7	33.3
Total Loans	5,833.6	5,833.5
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	8.1	9.3
Current Loans	765.6	776.8
Non-accrual Loans	2.0	1.7
Total Loans	775.7	787.8
Direct consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	2.4	2.0
Current Loans	191.2	198.1
Non-accrual Loans	0.5	0.5
Total Loans	194.1	200.6
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.8	2.1
Current Loans	75.4	79.7
Non-accrual Loans	0.0	0.0
Total Loans	77.2	81.8
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	12.3	13.4
Current Loans	1,032.2	1,054.6
Non-accrual Loans	2.5	2.2
Total Loans	1,047.0	1,070.2
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	9.8	10.8
Current Loans	1,317.0	1,283.7
Non-accrual Loans	15.3	15.8
Total Loans	1,342.1	1,310.3
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.1	2.4
Current Loans	229.0	249.4
Non-accrual Loans	2.6	3.0
Total Loans	234.7	254.8
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	1.8	1.6
Non-accrual Loans	0.0	0.0
Total Loans	1.8	1.6
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	63.6	54.8
Current Loans	8,350.5	8,361.3
Non-accrual Loans	45.1	54.3
Total Loans	8,459.2	8,470.4
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	34.0	33.3
Non-accrual Loans	0.0	0.0
Mortgage loans held for sale	34.0
Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	63.6	54.8
Current Loans	8,384.5	8,394.6
Non-accrual Loans	45.1	54.3
Total loans	8,493.2	8,503.7
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	12.6	10.4
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	1.2	1.6
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	2.2	1.0
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	0.4
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	3.4	3.0
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	4.6	8.8
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	4.0	2.2
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	24.6	24.4
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	6.5	6.8
30 to 59 Days Past Due | Direct consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	1.4	1.4
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	8.5	9.1
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	4.0	8.3
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	2.1
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	38.4	43.9
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	38.4	43.9
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	7.2	1.0
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	0.1
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	0.4
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	0.7	0.5
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	1.6	1.1
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	0.9	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	10.4	2.6
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	1.1	2.1
60 to 89 Days Past Due | Direct consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.7	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	0.5	0.4
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	2.3	3.0
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	4.1	1.2
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	1.3	0.3
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	18.1	7.1
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	18.1	7.1
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	2.5	0.8
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	0.2
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	0.2
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	0.2	0.2
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 | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	3.4	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	0.5	0.4
Equal to Greater than 90 Days Past Due | Direct consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total Loans 30 or More Days Past Due	0.3	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	0.8
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	1.5	1.3
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	1.7	1.3
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	0.5	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	7.1	3.8
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	$ 7.1	$ 3.8