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Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans | $ 7,614.5 | $ 7,567.7
Mortgage loans held for sale | 32.3 | 46.6
Total loans | 7,646.8 | 7,614.3
Commercial |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 5.9 | 3.7
Current Loans | 2,792.2 | 2,792.4
Non-accrual Loans | 23.8 | 26.8
Total Loans | 2,821.9 | 2,822.9
Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 1.2 | 7.9
Current Loans | 360.3 | 337.8
Non-accrual Loans | 3.1 | 3.0
Total Loans | 364.6 | 348.7
Residential |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 4.2 | 2.1
Current Loans | 225.7 | 236.4
Non-accrual Loans | 0.7 | 1.7
Total Loans | 230.6 | 240.2
Commercial |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 0.1 | 0.0
Current Loans | 136.9 | 115.6
Non-accrual Loans | 3.8 | 3.8
Total Loans | 140.8 | 119.4
Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 5.5 | 10.0
Current Loans | 722.9 | 689.8
Non-accrual Loans | 7.6 | 8.5
Total Loans | 736.0 | 708.3
Residential |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 10.9 | 15.1
Current Loans | 1,469.2 | 1,464.1
Non-accrual Loans | 6.5 | 8.2
Total Loans | 1,486.6 | 1,487.4
Agricultural |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 10.7 | 0.5
Current Loans | 141.6 | 154.3
Non-accrual Loans | 4.6 | 3.4
Total Loans | 156.9 | 158.2
Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 33.0 | 29.3
Current Loans | 5,125.9 | 5,100.6
Non-accrual Loans | 42.5 | 46.9
Total Loans | 5,201.4 | 5,176.8
Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 6.9 | 10.3
Current Loans | 774.1 | 772.6
Non-accrual Loans | 1.9 | 1.8
Total Loans | 782.9 | 784.7
Other consumer |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 1.5 | 2.2
Current Loans | 171.1 | 172.6
Non-accrual Loans | 0.4 | 0.3
Total Loans | 173.0 | 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 | 73.4 | 72.4
Non-accrual Loans | 0.0 | 0.0
Total Loans | 75.2 | 74.6
Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 10.2 | 14.7
Current Loans | 1,018.6 | 1,017.6
Non-accrual Loans | 2.3 | 2.1
Total Loans | 1,031.1 | 1,034.4
Commercial |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 12.6 | 6.3
Current Loans | 1,215.1 | 1,189.5
Non-accrual Loans | 16.4 | 19.6
Total Loans | 1,244.1 | 1,215.4
Agricultural |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 1.5 | 1.9
Current Loans | 130.5 | 133.5
Non-accrual Loans | 1.9 | 0.8
Total Loans | 133.9 | 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 | 4.0 | 4.9
Non-accrual Loans | 0.0 | 0.0
Total Loans | 4.0 | 4.9
Loans held for investment |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 57.3 | 52.2
Current Loans | 7,494.1 | 7,446.1
Non-accrual Loans | 63.1 | 69.4
Total Loans | 7,614.5 | 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 | 32.3 | 46.6
Non-accrual Loans | 0.0 | 0.0
Mortgage loans held for sale | 32.3 | 46.6
Total loans |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |
Total Loans 30 or More Days Past Due | 57.3 | 52.2
Current Loans | 7,526.4 | 7,492.7
Non-accrual Loans | 63.1 | 69.4
Total loans | 7,646.8 | 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 | 4.4 | 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 | 3.5 | 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 | 0.1 | 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 | 4.3 | 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 | 3.5 | 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 | 2.2 | 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 | 14.4 | 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 | 1.2 | 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 | 0.7 | 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 | 7.4 | 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 | 6.7 | 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 | 1.2 | 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 | 29.7 | 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 | 29.7 | 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 | 1.0 | 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 | 0.2 | 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 | 0.7 | 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 | 0.9 | 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 | 6.8 | 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 | 8.4 | 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 | 17.1 | 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 | 0.9 | 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 | 0.3 | 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 | 1.6 | 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 | 4.9 | 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 | 0.3 | 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 | 23.9 | 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 | 23.9 | 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 | 0.3 | 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 | 0.3 | 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 | 0.6 | 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 | 0.1 | 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 | 1.5 | 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 | 0.5 | 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 | 0.0 | 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 | 1.2 | 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 | 1.0 | 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 | 0.0 | 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 | 3.7 | 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 | $ 3.7 | $ 3.1

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- Definition Financing receivables that are current. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524 + Details
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- Definition Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524 + Details
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- Definition Recorded Investment in financing receivables that are past due at the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524 + Details
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- Definition Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522 + Details
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- Definition Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522 + Details
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