Variable Interest Entities - Summary of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Liability Recognized	$ 75,902	$ 38,235
Loans, net of unearned income	58,232	31,061
Trading securities:	1,176	1,346
Term borrowings	1,670	791
Securities available for sale at fair value	8,047	4,445
Variable Interest Entity, Not Primary Beneficiary | Single Asset Leasing Entity
Variable Interest Entity [Line Items]
Maximum Loss Exposure	18
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	338	238
Maximum loss exposure, contractual funding commitments	132	137
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	132	136
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	206	101
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	64	6
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	42	0
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	210	238
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	32	33
Loans, net of unearned income	112	112
Trading securities:	2	2
Term borrowings	82	81
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	82	81
Proprietary residential mortgage securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure		1
Proprietary residential mortgage securitizations | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized		0
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	7,063	4,538
Trading securities:	800	500
Securities available for sale at fair value	6,200	4,000
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	186	45
Maximum loss exposure, contractual funding commitments	10	2
Loans, net of unearned income	176	43
Commercial loan troubled debt restructurings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized	0	0
Sale-leaseback transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure		18
Sale-leaseback transaction | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liability Recognized		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	$ 287	$ 167