Variable Interest Entities - Summary Of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 27,990,048	[1]	$ 27,535,532	[1]	$ 27,249,793
Trading securities	1,681,727		1,448,168
Securities available-for-sale	4,616,322		4,626,470
Term borrowings	1,177,926		1,170,963
Low Income Housing Partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	152,048		156,056
Liability Recognized	79,691		80,427
Maximum loss exposure, contractual funding commitments	79,700		80,400
Low Income Housing Partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	72,400		75,600
Other Tax Credit Investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	8,961		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	269,635		270,585
Liability Recognized	0		0
On Balance Sheet Trust Preferred Securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	37,121		37,532
Liability Recognized	77,053		76,642
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	77,100		76,600
Proprietary & Agency Residential Mortgage Securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	1,397		1,524
Liability Recognized	0		0
Holdings Of Agency Mortgage Backed Securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	4,905,088		4,842,630
Liability Recognized	0		0
Trading securities	600,000		500,000
Securities available-for-sale	4,300,000		4,400,000
Commercial Loan Troubled Debt Restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	51,893		40,590
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments	300		2,300
Loans, net of unearned income	51,600		38,200
Sale Leaseback Transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	19,777		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] March 31, 2019 and December 31, 2018 include $24.1 million and $28.6 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.