Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($)	Sep. 30, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Liability Recognized	$ 73,740,000,000	$ 70,406,000,000
Trading securities	1,231,000,000	1,375,000,000
Term borrowings	1,157,000,000	1,597,000,000
Securities held to maturity	1,104,000,000	1,209,000,000
Securities available for sale at fair value	8,100,000,000	8,836,000,000
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	498,000,000	463,000,000
Maximum loss exposure, contractual funding commitments	155,000,000	154,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	155,000,000	154,000,000
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	343,000,000	309,000,000
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	81,000,000	85,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	65,000,000	67,000,000
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	173,000,000	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	27,000,000	27,000,000
Loans and leases	112,000,000
Loans and leases		112,000,000
Trading securities	2,000,000	2,000,000
Term borrowings	87,000,000	87,000,000
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	87,000,000	87,000,000
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	7,907,000,000	8,652,000,000
Trading securities	147,000,000	205,000,000
Securities held to maturity	1,300,000,000	1,400,000,000
Securities available for sale at fair value	6,400,000,000	7,100,000,000
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	39,000,000
Maximum loss exposure, contractual funding commitments	0
Commercial loan modifications to borrowers experiencing financial difficulty | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	0
Commercial loan modifications to borrowers experiencing financial difficulty | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	39,000,000
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure		53,000,000
Maximum loss exposure, contractual funding commitments		0
Commercial loan troubled debt restructurings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized		0
Commercial loan troubled debt restructurings | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments		53,000,000
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