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Variable Interest Entities - Schedule of VIEs Not Consolidated by FHN (Details) - USD ($) | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Variable Interest Entity [Line Items] |  |
Liability Recognized | $ 73,948,000,000 | $ 73,041,000,000
Trading securities | 2,070,000,000 | 1,387,000,000
Term borrowings | 1,328,000,000 | 1,195,000,000
Securities held to maturity | 1,079,000,000 | 1,083,000,000
Securities available for sale at fair value | 8,102,000,000 | 7,896,000,000
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 | 89,000,000 | 88,000,000
Holdings of agency mortgage-backed securities |  |
Variable Interest Entity [Line Items] |  |
Trading securities | 538,000,000 | 278,000,000
Securities held to maturity | 1,200,000,000 | 1,300,000,000
Securities available for sale at fair value | 6,500,000,000 | 6,500,000,000
Commercial loan troubled debt restructurings |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, contractual funding commitments | 1,000,000 | 0
Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, contractual funding commitments | 258,000,000 | 222,000,000
Other Assets | Commercial loan troubled debt restructurings |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments |  | 381,000,000
Other Assets | Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments | 431,000,000 | 395,000,000
Other Assets | Commercial loan modifications to borrowers experiencing financial difficulty |  |
Variable Interest Entity [Line Items] |  |
Maximum loss exposure, current investments | 495,000,000 |
Low income housing partnerships |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 689,000,000 | 617,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 258,000,000 | 222,000,000
Other tax credit investments |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 97,000,000 | 90,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 81,000,000 | 73,000,000
Small issuer trust preferred holdings |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 166,000,000 | 171,000,000
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 | 25,000,000 | 26,000,000
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
Liability Recognized | 89,000,000 | 88,000,000
Holdings of agency mortgage-backed securities |  |
Variable Interest Entity [Line Items] |  |
Maximum Loss Exposure | 8,292,000,000 | 8,017,000,000
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 | 495,000,000 | 381,000,000
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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