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Investment Securities - Pledged Securities and Concentration Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0.0	$ 0.0
Collateral Dependent Loans
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	4,400.0	3,800.0
Collateral Dependent Loans | Borrowing capacity
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	2,400.0	2,100.0
Collateral Dependent Loans | Public deposits
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	2,000.0	1,700.0
Collateral Dependent Loans | Other financial transactions
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	$ 49.5	$ 19.3

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- Definition

Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Fair value of investment subject to concentration of credit or market risk or both.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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