Variable Interest Entities - Schedule of VIEs Not Consolidated by FHN (Details) - USD ($)	Mar. 31, 2026	Dec. 31, 2025
Variable Interest Entity [Line Items]
Liability Recognized	$ 74,667,000,000	$ 74,734,000,000
Trading securities	1,812,000,000	1,904,000,000
Term borrowings	1,318,000,000	1,321,000,000
Securities held to maturity	1,056,000,000	1,073,000,000
Securities available for sale at fair value	8,146,000,000	8,165,000,000
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum loss exposure, contractual funding commitments	230,000,000	260,000,000
On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Loans and leases	113,000,000	113,000,000
Trading securities	1,000,000	1,000,000
Term borrowings	89,000,000	89,000,000
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Trading securities	702,000,000	678,000,000
Securities held to maturity	1,200,000,000	1,200,000,000
Securities available for sale at fair value	6,300,000,000	6,500,000,000
Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Maximum loss exposure, contractual funding commitments	1,000,000	1,000,000
Other Assets | Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	483,000,000	473,000,000
Other Assets | Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	390,000,000	477,000,000
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	713,000,000	733,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	230,000,000	260,000,000
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	91,000,000	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	166,000,000	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	25,000,000	25,000,000
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	89,000,000	89,000,000
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	8,224,000,000	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	391,000,000	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
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