Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 31,260,833	[1]	$ 27,535,532	[1]	$ 27,350,214
Trading securities	1,395,043		1,448,168
Term borrowings	1,195,096		1,170,963
Securities available-for-sale	4,415,845		4,626,470
Low Income Housing Partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	183,975		156,056
Liability Recognized	96,637		80,427
Maximum loss exposure, contractual funding commitments	96,600		80,400
Low Income Housing Partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	87,300		75,600
Other Tax Credit Investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	9,226		3,619
Liability Recognized	0		0
Other Tax Credit Investments | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	2,700		2,700
Small Issuer Trust Preferred Holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	237,422		270,585
Liability Recognized	0		0
On Balance Sheet Trust Preferred Securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	33,384		37,532
Liability Recognized	80,790		76,642
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	80,800		76,600
Proprietary & Agency Residential Mortgage Securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	1,098		1,524
Liability Recognized	0		0
Holdings Of Agency Mortgage Backed Securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	4,762,162		4,842,630
Liability Recognized	0		0
Trading securities	700,000		500,000
Securities available-for-sale	4,100,000		4,400,000
Commercial Loan Troubled Debt Restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	47,940		40,590
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments	1,200		2,300
Loans, net of unearned income	46,700		38,200
Sale Leaseback Transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	18,170		16,327
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] September 30, 2019 and December 31, 2018 include $18.2 million and $28.6 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.