Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Total liabilities	$ 80,005	$ 75,902
Trading securities	1,319	1,176
Term borrowings	1,584	1,670
Securities available-for-sale	8,494	8,047
Securities held to maturity, at fair value	303	10
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	349	338
Maximum loss exposure, contractual funding commitments	116	132
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	116	132
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	233	206
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	63	64
Other tax credit investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	42	42
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	202	210
Small issuer trust preferred holdings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	0	0
On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	30	32
Loans and leases	112	112
Trading securities	2	2
Term borrowings	84	82
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	84	82
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	7,380	7,063
Trading securities	514	845
Securities available-for-sale	6,600	6,200
Securities held to maturity, at fair value	304
Holdings of agency mortgage-backed securities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	0	0
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	117	186
Maximum loss exposure, contractual funding commitments	4	10
Loans and leases	113	176
Commercial loan troubled debt restructurings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	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]
Total liabilities	$ 184	$ 287