Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($)	Sep. 30, 2022	Dec. 31, 2021
Variable Interest Entity [Line Items]
Liability Recognized	$ 72,016,000,000	$ 80,598,000,000
Trading securities	1,421,000,000	1,601,000,000
Term borrowings	1,597,000,000	1,590,000,000
Securities held to maturity, at fair value	1,222,000,000	705,000,000
Securities available-for-sale	8,718,000,000	8,707,000,000
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	445,000,000	382,000,000
Maximum loss exposure, contractual funding commitments	160,000,000	129,000,000
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	160,000,000	129,000,000
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	285,000,000	253,000,000
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	87,000,000	77,000,000
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	67,000,000	56,000,000
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	172,000,000	195,000,000
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	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	8,785,000,000	8,550,000,000
Trading securities	296,000,000	526,000,000
Securities held to maturity, at fair value	1,400,000,000	712,000,000
Securities available-for-sale	7,100,000,000	7,300,000,000
Holdings of agency mortgage-backed securities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	0	0
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	55,000,000	98,000,000
Maximum loss exposure, contractual funding commitments	0	4,000,000
Loans and leases	55,000,000	94,000,000
Commercial loan troubled debt restructurings | 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