Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Total liabilities	$ 79,206	$ 75,902
Loans and leases	58,600	58,232
Trading securities	1,076	1,176
Term borrowings	1,671	1,670
Securities available-for-sale	8,351	8,047
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	345	338
Maximum loss exposure, contractual funding commitments	126	132
Low income housing partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	126	132
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	219	206
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	68	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	210	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	32	32
Loans and leases	112	112
Trading securities	2	2
Term borrowings	83	82
On-balance sheet trust preferred securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	83	82
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	7,053	7,063
Trading securities	600	800
Securities available-for-sale	6,500	6,200
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	169	186
Maximum loss exposure, contractual funding commitments	3	10
Loans and leases	166	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	$ 287	$ 287