Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Jun. 30, 2020		Dec. 31, 2019		Jun. 30, 2019
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 32,708,937	[1]	$ 31,061,111	[1]	$ 29,712,810
Trading securities	1,116,450		1,346,207
Term borrowings	2,032,476		791,368
Securities available-for-sale	5,476,156		4,445,403
Low Income Housing Partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	248,712		237,668
Liability Recognized	124,318		136,404
Maximum loss exposure, contractual funding commitments	124,300		136,400
Low Income Housing Partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	124,400		101,300
Other Tax Credit Investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	5,959		6,282
Liability Recognized	0		0
Small Issuer Trust Preferred Holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	209,177		238,397
Liability Recognized	0		0
On Balance Sheet Trust Preferred Securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	32,144		33,265
Liability Recognized	82,030		80,908
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	82,000		80,900
Proprietary & Agency Residential Mortgage Securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	628		941
Liability Recognized	0		0
Holdings Of Agency Mortgage Backed Securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	4,561,144		4,537,685
Liability Recognized	0		0
Trading securities	700,000		500,000
Securities available-for-sale	3,900,000		4,000,000
Commercial Loan Troubled Debt Restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	43,432		45,169
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments			1,800
Loans, net of unearned income	43,400		43,400
Sale Leaseback Transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	18,052		18,111
Liability Recognized	0		0
Proprietary Trust Preferred Issuances
Variable Interest Entity [Line Items]
Maximum Loss Exposure	0		0
Liability Recognized	$ 167,014		$ 167,014

[1] June 30, 2020 and December 31, 2019 include $16.9 million and $18.8 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.