Variable Interest Entities - Summary of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Variable Interest Entity [Line Items]
Liability Recognized	$ 70,406	$ 80,598
Trading securities:	1,375	1,601
Securities held to maturity	1,371	712
Term borrowings	1,597	1,590
Fair Value	1,209	705
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	463	382
Maximum loss exposure, contractual funding commitments	154	129
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	154	129
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	309	253
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	85	77
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	67	56
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	171	195
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	27
Loans, net of unearned income	112	112
Trading securities:	2	2
Term borrowings	87	87
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	87	87
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	8,652	8,550
Trading securities:	205	526
Securities held to maturity	1,400
Fair Value		712
Securities available for sale at fair value	7,100	7,300
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	53	98
Maximum loss exposure, contractual funding commitments		4
Commercial loan troubled debt restructurings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	0	0
Commercial loan troubled debt restructurings | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	53	94
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	$ 167