Variable Interest Entities - Summary of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Liability Recognized	$ 80,598	$ 75,902
Trading securities:	1,601	1,176
Securities held to maturity	712	10
Term borrowings	1,590	1,670
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	382	338
Maximum loss exposure, contractual funding commitments	129	132
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	129	132
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	253	206
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	77	64
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	56	42
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	195	210
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	32
Loans, net of unearned income	112	112
Trading securities:	2	2
Term borrowings	87	82
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	87	82
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	8,550	7,063
Trading securities:	526	845
Securities held to maturity	712
Securities available for sale at fair value	7,300	6,200
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	98	186
Maximum loss exposure, contractual funding commitments	4	10
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	94	176
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	$ 287