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Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2018 | Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | $ 72.1 | $ 76.2 |  |
Provision charged to operating expense | 2.1 | 1.7 |  |
Less loans charged-off | 5.2 | 3.8 |  |
Add back recoveries of loans previously charged-off | 4.5 | 2.1 |  |
Ending balance | 73.5 | 76.2 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |
Loans individually evaluated for impairment |  |  | $ 10.6 | $ 10.8
Loans collectively evaluated for impairment |  |  | 62.9 | 61.3
Allowance for loan losses | 72.1 | 76.2 | 73.5 | 72.1
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 76.5 | 83.2
Individually evaluated for impairment |  |  | 7,538.0 | 7,484.5
Total loans |  |  | 7,614.5 | 7,567.7
Allowance for credit losses due to subsequent impairment |  |  | 1.0 |
Real Estate |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | 31.6 | 28.6 |  |
Provision charged to operating expense | 1.8 | 4.4 |  |
Less loans charged-off | 0.8 | 0.4 |  |
Add back recoveries of loans previously charged-off | 1.2 | 0.5 |  |
Ending balance | 33.8 | 33.1 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |
Loans individually evaluated for impairment |  |  | 6.0 | 6.2
Loans collectively evaluated for impairment |  |  | 27.8 | 25.4
Allowance for loan losses | 31.6 | 28.6 | 33.8 | 31.6
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 54.1 | 58.3
Individually evaluated for impairment |  |  | 5,147.3 | 5,118.5
Total loans |  |  | 5,201.4 | 5,176.8
Consumer |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | 8.7 | 7.7 |  |
Provision charged to operating expense | 0.7 | 2.0 |  |
Less loans charged-off | 2.7 | 2.6 |  |
Add back recoveries of loans previously charged-off | 1.1 | 1.2 |  |
Ending balance | 7.8 | 8.3 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |
Loans individually evaluated for impairment |  |  | 0.0 | 0.0
Loans collectively evaluated for impairment |  |  | 7.8 | 8.7
Allowance for loan losses | 8.7 | 7.7 | 7.8 | 8.7
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 0.0 | 0.0
Individually evaluated for impairment |  |  | 1,031.1 | 1,034.4
Total loans |  |  | 1,031.1 | 1,034.4
Commercial |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | 30.6 | 38.1 |  |
Provision charged to operating expense | (0.7) | (4.1) |  |
Less loans charged-off | 1.7 | 0.8 |  |
Add back recoveries of loans previously charged-off | 2.0 | 0.4 |  |
Ending balance | 30.2 | 33.6 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |
Loans individually evaluated for impairment |  |  | 4.5 | 4.4
Loans collectively evaluated for impairment |  |  | 25.7 | 26.2
Allowance for loan losses | 30.6 | 38.1 | 30.2 | 30.6
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 20.4 | 23.8
Individually evaluated for impairment |  |  | 1,223.7 | 1,191.6
Total loans |  |  | 1,244.1 | 1,215.4
Agriculture |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | 1.2 | 1.8 |  |
Provision charged to operating expense | 0.3 | (0.6) |  |
Less loans charged-off | 0.0 | 0.0 |  |
Add back recoveries of loans previously charged-off | 0.2 | 0.0 |  |
Ending balance | 1.7 | 1.2 |  |
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] |  |  |  |
Loans individually evaluated for impairment |  |  | 0.1 | 0.2
Loans collectively evaluated for impairment |  |  | 1.6 | 1.0
Allowance for loan losses | 1.2 | 1.8 | 1.7 | 1.2
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 2.0 | 1.1
Individually evaluated for impairment |  |  | 131.9 | 135.1
Total loans |  |  | 133.9 | 136.2
Other |  |  |  |
Allowance for Loan and Lease Losses [Roll Forward] |  |  |  |
Beginning balance | 0.0 | 0.0 |  |
Provision charged to operating expense | 0.0 | 0.0 |  |
Less loans charged-off | 0.0 | 0.0 |  |
Add back recoveries of loans previously charged-off | 0.0 | 0.0 |  |
Ending balance | 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
Loans held for investment: |  |  |  |
Individually evaluated for impairment |  |  | 0.0 | 0.0
Individually evaluated for impairment |  |  | 4.0 | 4.9
Total loans |  |  | $ 4.0 | $ 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://www.xbrl.org/2003/role/presentationRef -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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- Definition Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition For loans and debt securities, 1) for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable, and 2) for which it is further probable that the investor will not be able to collect all cash flows expected at acquisition (plus any additional cash flows expected due to changes in estimate after acquisition), the allowance for credit losses recorded to reflect the impairment since acquisition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5275-111524 + Details
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- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for 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 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544 + Details
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