Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019		Mar. 31, 2019
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 33,378,303	[1]	$ 31,061,111	[1]	$ 27,990,048
Trading securities	1,877,514		1,346,207
Term borrowings	792,751		791,368
Securities available-for-sale	4,544,907		4,445,403
Low Income Housing Partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	241,435		237,668
Liability Recognized	123,720		136,404
Maximum loss exposure, contractual funding commitments	123,700		136,400
Low Income Housing Partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	117,700		101,300
Other Tax Credit Investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	6,161		6,282
Liability Recognized	0		0
Small Issuer Trust Preferred Holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	234,214		238,397
Liability Recognized	0		0
On Balance Sheet Trust Preferred Securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	32,261		33,265
Liability Recognized	81,912		80,908
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	81,900		80,900
Proprietary & Agency Residential Mortgage Securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	785		941
Liability Recognized	0		0
Holdings Of Agency Mortgage Backed Securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	5,126,372		4,537,685
Liability Recognized	0		0
Trading securities	1,100,000		500,000
Securities available-for-sale	4,000,000		4,000,000
Commercial Loan Troubled Debt Restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	42,109		45,169
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments	500		1,800
Loans, net of unearned income	41,600		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] March 31, 2020 and December 31, 2019 include $20.8 million and $18.8 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.