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Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019
Entity Information [Line Items] |  |  |  |  |  |  |  |  |
Off-Balance Sheet, Credit Loss, Liability | $ 3.2 | $ 3.5 | $ 3.2 | $ 3.5 | $ 3.4 | $ 3.7 | $ 2.3 |  | $ 0.0
Provision for (reversal of) credit loss expense | (0.2) | (4.0) |  | (52.5) |  |  |  |  |
Credit Extension Commitments | 2,558.6 |  | 2,558.6 |  |  | 2,280.0 |  |  |
Unused Credit Card Lines |  |  |  |  |  |  |  |  |
Entity Information [Line Items] |  |  |  |  |  |  |  |  |
Credit Extension Commitments | 682.5 |  | 682.5 |  |  | 682.8 |  |  |
Standby Letter of Credit |  |  |  |  |  |  |  |  |
Entity Information [Line Items] |  |  |  |  |  |  |  |  |
Credit Extension Commitments | 59.8 |  | 59.8 |  |  | 59.0 |  |  |
Cumulative effect, period of adoption, adjusted balance |  |  |  |  |  |  |  |  |
Entity Information [Line Items] |  |  |  |  |  |  |  |  |
Off-Balance Sheet, Credit Loss, Liability |  | 2.3 |  | 2.3 |  | $ 0.0 |  | $ 0.0 |
Provision for (reversal of) credit loss expense | $ (0.2) | $ 1.2 | $ (0.5) | $ 1.2 |  |  |  |  |

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- Definition Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121611286&loc=d3e11927-110247 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82919202-210446 + Details
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- Definition Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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