Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Variable Interest Entity [Line Items]
Liability Recognized	$ 79,964	$ 80,598
Trading securities	1,823	1,601
Term borrowings	1,591	1,590
Securities held to maturity, at fair value	650	705
Securities available-for-sale	9,242	8,707
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	410	382
Maximum loss exposure, contractual funding commitments	149	129
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	149	129
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	261	253
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	79	77
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	62	56
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	195	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 and leases	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	9,141	8,550
Trading securities	678	526
Securities held to maturity, at fair value	701	712
Securities available-for-sale	7,800	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	93	98
Maximum loss exposure, contractual funding commitments	9	4
Loans and leases	84	94
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	$ 167