Variable Interest Entities - Summary of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 31,061,111	[1]	$ 27,535,532	[1]	$ 27,658,929
Trading securities	1,346,207		1,448,168
Term borrowings	791,368		1,170,963
Securities available-for-sale (Note 3)	4,445,403		4,626,470
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	237,668		156,056
Liability Recognized	136,404		80,427
Maximum loss exposure, contractual funding commitments	136,400		80,400
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	101,300		75,600
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	6,282		3,619
Liability Recognized	0		0
Other tax credit investments | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments			2,700
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	238,397		270,585
Liability Recognized	0		0
On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	33,265		37,532
Liability Recognized	80,908		76,642
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	80,900		76,600
Proprietary residential mortgage securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	941		1,524
Liability Recognized	0		0
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	4,537,685		4,842,630
Liability Recognized	0		0
Trading securities	500,000		500,000
Securities available-for-sale (Note 3)	4,000,000		4,400,000
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	45,169		40,590
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments	1,800		2,300
Loans, net of unearned income	43,400		38,200
Sale-leaseback transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	18,111		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] December 31, 2019 and 2018 include $18.8 million and $28.6 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.