Investment Securities - Pledged Securities and Concentration Narrative (Details)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2024 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Concentration of risk
Number of securities with intent to sell	0		0
Credit losses recognized on securities	$ 0	$ 0	$ 0
Collateral Dependent Loans
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	4,100,000,000	4,100,000,000	5,000,000,000.0
Collateral Dependent Loans \| Public deposits
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	2,000,000,000.0	2,000,000,000.0	2,600,000,000
Collateral Dependent Loans \| Other financial transactions
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	19,500,000	19,500,000	183,000,000.0
Borrowing Capacity
Financial Instruments Owned and Pledged as Collateral
Investment securities pledged as collateral	2,100,000,000	2,100,000,000	2,300,000,000
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0	$ 0	$ 0

Represents the number of securities with intent to sell.

No definition available.

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 80
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480078/944-80-55-14

Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(6))
-Publisher FASB
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+ Details

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- Definition

Amount of increase (decrease) in 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).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-9

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- References

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
-Section S99
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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