Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Proceeds from calls and sales of investment securities
Proceeds from calls of investment securities	$ 73.5	$ 8.3	$ 1.7
Proceeds from sales of investment securities	264.3	551.6	0.0
Gross realized gains and losses on investment securities
Gross realized gains on sales of investment securities	0.0	40.8	0.0
Gross realized losses on sales of investment securities	26.2	40.0	0.0
Income tax benefit or (Expense) related to net realized loss on sale of investment securities	(7.0)	0.2	0.0
Interest income from taxable and nontaxable investment securities
Interest income from taxable investment securities	110.9	134.5	128.7
Interest income from non-taxable investment securities	$ 12.8	$ 13.2	$ 13.8

Amount of realized gain on investment in debt security and equity securities.

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Amount of realized loss on investment in debt security and equity securities.

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The cash inflow associated with calls on debt securities.

+ References

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities exempt from state, federal and other income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph a
-Publisher SEC

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

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-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph b
-Subparagraph (1)(Instruction 4)
-Publisher SEC

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-SubTopic 10
-Topic 220
-Publisher FASB
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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481830/320-10-45-11

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-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

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-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
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-Subparagraph (a)
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-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

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-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-11

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