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Variable Interest Entities - Schedule of VIEs Not Consolidated by FHN (Details) - USD ($) | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Variable Interest Entity [Line Items] |  |
Liability Recognized | $ 73,319,000,000 | $ 72,370,000,000
Trading securities | 1,549,000,000 | 1,412,000,000
Term borrowings | 1,202,000,000 | 1,150,000,000
Securities held to maturity | 1,144,000,000 | 1,161,000,000
Securities available for sale at fair value | 8,248,000,000 | 8,391,000,000
On-balance sheet trust preferred securitization |  |
Variable Interest Entity [Line Items] |  |
Loans and leases | 113,000,000 | 113,000,000
Trading securities | 2,000,000 | 2,000,000
Term borrowings | 88,000,000 | 88,000,000
Holdings of agency mortgage-backed securities |  |
Variable Interest Entity [Line Items] |  |
Trading securities | 428,000,000 | 450,000,000
Securities held to maturity | 1,300,000,000 | 1,300,000,000
Securities available for sale at fair value | 6,600,000,000 | 6,600,000,000
Commercial loan troubled debt restructurings |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, contractual funding commitments |  | 0
Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, contractual funding commitments | 201,000,000 | 223,000,000
Commercial loan modifications to borrowers experiencing financial difficulty |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, contractual funding commitments | 0 |
Other Assets | Commercial loan troubled debt restructurings |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments |  | 129,000,000
Other Assets | Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments | 391,000,000 | 364,000,000
Other Assets | Commercial loan modifications to borrowers experiencing financial difficulty |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments | 152,000,000 |
Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 592,000,000 | 587,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 201,000,000 | 223,000,000
Other tax credit investments |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 98,000,000 | 79,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 82,000,000 | 64,000,000
Small issuer trust preferred holdings |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 171,000,000 | 173,000,000
Small issuer trust preferred holdings | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 0 | 0
On-balance sheet trust preferred securitization |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 26,000,000 | 26,000,000
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 88,000,000 | 88,000,000
Holdings of agency mortgage-backed securities |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 8,263,000,000 | 8,402,000,000
Holdings of agency mortgage-backed securities | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 0 | 0
Commercial loan modifications to borrowers experiencing financial difficulty |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 152,000,000 | 129,000,000
Commercial loan modifications to borrowers experiencing financial difficulty | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 0 | 0
Proprietary trust preferred issuances |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 0 | 0
Proprietary trust preferred issuances | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | $ 167,000,000 | $ 167,000,000

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