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Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | $ 74.1 | $ 75.7 | $ 72.1 | $ 76.2 |  |
Provision charged to operating expense | 2.0 | 3.4 | 7.0 | 7.5 |  |
Less loans charged-off | (4.8) | (7.4) | (15.1) | (15.7) |  |
Add back recoveries of loans previously charged-off | 2.3 | 2.9 | 9.6 | 6.6 |  |
Ending balance | 73.6 | 74.6 | 73.6 | 74.6 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | $ 8.2 | $ 10.8
Loans collectively evaluated for impairment |  |  |  |  | 65.4 | 61.3
Allowance for loan losses | 74.1 | 75.7 | 72.1 | 76.2 | 73.6 | 72.1
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 72.4 | 83.2
Individually evaluated for impairment |  |  |  |  | 8,407.9 | 7,484.5
Total loans |  |  |  |  | 8,480.3 | 7,567.7
Allowance for credit losses due to subsequent impairment |  |  |  |  | 0.6 | 1.0
Real Estate |  |  |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | 33.9 | 33.5 | 31.6 | 28.6 |  |
Provision charged to operating expense | (0.9) | (0.7) | 1.4 | 4.8 |  |
Less loans charged-off | (0.7) | (1.7) | (2.8) | (3.0) |  |
Add back recoveries of loans previously charged-off | 0.6 | 0.6 | 2.7 | 1.3 |  |
Ending balance | 32.9 | 31.7 | 32.9 | 31.7 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | 3.6 | 6.2
Loans collectively evaluated for impairment |  |  |  |  | 29.3 | 25.4
Allowance for loan losses | 33.9 | 33.5 | 31.6 | 28.6 | 32.9 | 31.6
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 49.8 | 58.3
Individually evaluated for impairment |  |  |  |  | 5,747.0 | 5,118.5
Total loans |  |  |  |  | 5,796.8 | 5,176.8
Consumer |  |  |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | 7.8 | 7.6 | 8.7 | 7.7 |  |
Provision charged to operating expense | 2.4 | 2.6 | 4.7 | 5.2 |  |
Less loans charged-off | (3.0) | (3.1) | (8.6) | (8.1) |  |
Add back recoveries of loans previously charged-off | 1.2 | 1.0 | 3.6 | 3.3 |  |
Ending balance | 8.4 | 8.1 | 8.4 | 8.1 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | 0.0 | 0.0
Loans collectively evaluated for impairment |  |  |  |  | 8.4 | 8.7
Allowance for loan losses | 7.8 | 7.6 | 8.7 | 7.7 | 8.4 | 8.7
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 0.0 | 0.0
Individually evaluated for impairment |  |  |  |  | 1,078.3 | 1,034.4
Total loans |  |  |  |  | 1,078.3 | 1,034.4
Commercial |  |  |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | 30.6 | 33.1 | 30.6 | 38.1 |  |
Provision charged to operating expense | 0.5 | 1.5 | 0.5 | (2.2) |  |
Less loans charged-off | (1.1) | (2.6) | (3.7) | (4.5) |  |
Add back recoveries of loans previously charged-off | 0.5 | 1.3 | 3.1 | 1.9 |  |
Ending balance | 30.5 | 33.3 | 30.5 | 33.3 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | 4.4 | 4.4
Loans collectively evaluated for impairment |  |  |  |  | 26.1 | 26.2
Allowance for loan losses | 30.6 | 33.1 | 30.6 | 38.1 | 30.5 | 30.6
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 20.4 | 23.8
Individually evaluated for impairment |  |  |  |  | 1,316.8 | 1,191.6
Total loans |  |  |  |  | 1,337.2 | 1,215.4
Agriculture |  |  |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | 1.8 | 1.5 | 1.2 | 1.8 |  |
Provision charged to operating expense | 0.0 | 0.0 | 0.4 | (0.3) |  |
Less loans charged-off | 0.0 | 0.0 | 0.0 | (0.1) |  |
Add back recoveries of loans previously charged-off | 0.0 | 0.0 | 0.2 | 0.1 |  |
Ending balance | 1.8 | 1.5 | 1.8 | 1.5 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | 0.2 | 0.2
Loans collectively evaluated for impairment |  |  |  |  | 1.6 | 1.0
Allowance for loan losses | 1.8 | 1.5 | 1.2 | 1.8 | 1.8 | 1.2
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 2.2 | 1.1
Individually evaluated for impairment |  |  |  |  | 262.6 | 135.1
Total loans |  |  |  |  | 264.8 | 136.2
Other |  |  |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |  |  |
Beginning balance | 0.0 | 0.0 | 0.0 | 0.0 |  |
Provision charged to operating expense | 0.0 | 0.0 | 0.0 | 0.0 |  |
Less loans charged-off | 0.0 | 0.0 | 0.0 | 0.0 |  |
Add back recoveries of loans previously charged-off | 0.0 | 0.0 | 0.0 | 0.0 |  |
Ending balance | 0.0 | 0.0 | 0.0 | 0.0 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |  |  |
Loans individually evaluated for impairment |  |  |  |  | 0.0 | 0.0
Loans collectively evaluated for impairment |  |  |  |  | 0.0 | 0.0
Allowance for loan losses | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | 0.0 | 0.0
Loans held for investment: |  |  |  |  |  |
Individually evaluated for impairment |  |  |  |  | 0.0 | 0.0
Individually evaluated for impairment |  |  |  |  | 3.2 | 4.9
Total loans |  |  |  |  | $ 3.2 | $ 4.9

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- Definition Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 + Details
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