Variable Interest Entities - Schedule of VIEs Not Consolidated by FHN (Details) - USD ($)	Jun. 30, 2025	Dec. 31, 2024
Variable Interest Entity [Line Items]
Liability Recognized	$ 72,827,000,000	$ 73,041,000,000
Trading securities	1,430,000,000	1,387,000,000
Term borrowings	1,342,000,000	1,195,000,000
Securities held to maturity	1,083,000,000	1,083,000,000
Securities available for sale at fair value	8,117,000,000	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	268,000,000	278,000,000
Securities held to maturity	1,200,000,000	1,300,000,000
Securities available for sale at fair value	6,700,000,000	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	282,000,000	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	415,000,000	395,000,000
Other Assets | Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	557,000,000
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	697,000,000	617,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	282,000,000	222,000,000
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	88,000,000	90,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	73,000,000	73,000,000
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	165,000,000	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	8,191,000,000	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	557,000,000	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