v3.26.1
Variable Interest Entities - Schedule of VIEs Not Consolidated by FHN (Details) - USD ($)
Jun. 30, 2026
Dec. 31, 2025
Variable Interest Entity [Line Items]    
Liability Recognized $ 74,974,000,000 $ 74,734,000,000
Maximum loss exposure, current investments   477,000,000
Trading securities 1,417,000,000 1,904,000,000
Term borrowings 1,321,000,000 1,321,000,000
Securities held to maturity 1,038,000,000 1,073,000,000
Securities available for sale at fair value 7,873,000,000 8,165,000,000
Variable Interest Entity, Not Primary Beneficiary | Holdings of agency mortgage-backed securities    
Variable Interest Entity [Line Items]    
Trading securities 376,000,000 678,000,000
Securities held to maturity 1,200,000,000 1,200,000,000
Securities available for sale at fair value 6,100,000,000 6,500,000,000
Low income housing partnerships    
Variable Interest Entity [Line Items]    
Maximum Loss Exposure 730,000,000 733,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Liability Recognized 244,000,000 260,000,000
Maximum loss exposure, current investments 486,000,000 473,000,000
Maximum loss exposure, contractual funding commitments 244,000,000 260,000,000
Other tax credit investments    
Variable Interest Entity [Line Items]    
Maximum Loss Exposure 90,000,000 92,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Liability Recognized 74,000,000 74,000,000
Small issuer trust preferred holdings    
Variable Interest Entity [Line Items]    
Maximum Loss Exposure 154,000,000 166,000,000
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 23,000,000 25,000,000
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Liability Recognized 91,000,000 89,000,000
Loans and leases 113,000,000 113,000,000
Trading securities 1,000,000 1,000,000
Term borrowings 91,000,000 89,000,000
Holdings of agency mortgage-backed securities    
Variable Interest Entity [Line Items]    
Maximum Loss Exposure 7,661,000,000 8,405,000,000
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 321,000,000 478,000,000
Commercial loan modifications to borrowers experiencing financial difficulty | Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Liability Recognized 0 0
Maximum loss exposure, current investments 320,000,000  
Maximum loss exposure, contractual funding commitments 1,000,000 1,000,000
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