Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Total liabilities	$ 74,885,437	$ 38,234,892
Loans and leases (including $12,372 and $— at fair value, respectively)	59,706,531	31,061,111
Trading securities	1,386,069	1,346,207
Term borrowings	2,161,870	791,368
Securities available-for-sale	7,995,872	4,445,403
Low Income Housing Partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	338,558	237,668
Maximum loss exposure, contractual funding commitments	166,900	136,400
Low Income Housing Partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	166,931	136,404
Low Income Housing Partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	171,700	101,300
Other Tax Credit Investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	66,172	6,282
Other Tax Credit Investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	34,993	0
Small Issuer Trust Preferred Holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	209,442	238,397
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	32,031	33,265
Loans and leases (including $12,372 and $— at fair value, respectively)	112,500	112,500
Trading securities	1,700	1,700
Term borrowings	82,100	80,900
On Balance Sheet Trust Preferred Securitization | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	82,143	80,908
Proprietary & Agency Residential Mortgage Securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	471	941
Proprietary & Agency Residential Mortgage Securitizations | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	0	0
Holdings Of Agency Mortgage Backed Securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	7,386,169	4,537,685
Trading securities	900,000	500,000
Securities available-for-sale	6,500,000	4,000,000
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	177,949	45,169
Maximum loss exposure, contractual funding commitments	5,100	1,800
Loans and leases (including $12,372 and $— at fair value, respectively)	172,800	43,400
Commercial Loan Troubled Debt Restructurings | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total liabilities	0	0
Sale Leaseback Transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	17,993	18,111
Sale Leaseback Transaction | 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	$ 287,124	$ 167,014