Variable Interest Entities - Schedule of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Liability Recognized	$ 72,370	$ 70,406
Loans and leases	61,292	58,102
Trading securities:	1,412	1,375
Term borrowings	1,150	1,597
Securities held to maturity	1,323	1,371
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	587	463
Maximum loss exposure, contractual funding commitments	223	154
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	79	85
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	173	171
On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	26	27
Loans and leases	113	112
Trading securities:	2	2
Term borrowings	88	87
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	8,402	8,652
Trading securities:	450	205
Securities held to maturity	1,300	1,400
Securities available for sale at fair value	6,600	7,100
Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Maximum Loss Exposure	129
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure		53
Proprietary trust preferred issuances
Variable Interest Entity [Line Items]
Maximum Loss Exposure	0	0
Variable Interest Entity, Not Primary Beneficiary | Low income housing partnerships
Variable Interest Entity [Line Items]
Liability Recognized	223	154
Variable Interest Entity, Not Primary Beneficiary | Other tax credit investments
Variable Interest Entity [Line Items]
Liability Recognized	64	67
Variable Interest Entity, Not Primary Beneficiary | Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Liability Recognized	0	0
Variable Interest Entity, Not Primary Beneficiary | On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Liability Recognized	88	87
Variable Interest Entity, Not Primary Beneficiary | Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Liability Recognized	0	0
Variable Interest Entity, Not Primary Beneficiary | Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Liability Recognized	0
Variable Interest Entity, Not Primary Beneficiary | Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Liability Recognized		0
Variable Interest Entity, Not Primary Beneficiary | Proprietary trust preferred issuances
Variable Interest Entity [Line Items]
Liability Recognized	167	167
Other Assets | Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	364	309
Other Assets | Commercial loan modifications to borrowers experiencing financial difficulty
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	$ 129
Other Assets | Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments		$ 53