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