Variable Interest Entities - Summary of VIE Not Consolidated By FHN (Details) - USD ($) $ in Thousands	Sep. 30, 2018		Dec. 31, 2017		Sep. 30, 2017
Variable Interest Entity [Line Items]
Loans, net of unearned income	$ 27,350,214	[1]	$ 27,658,929	[1]	$ 20,166,091
Trading securities	1,930,991		1,416,345
Term borrowings	1,200,134		1,218,097
Low income housing partnerships
Variable Interest Entity [Line Items]
Maximum Loss Exposure	118,355		94,798
Liability Recognized	52,148		33,348
Maximum loss exposure, contractual funding commitments	52,100		33,300
Low income housing partnerships | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	66,200		61,500
Other tax credit investments
Variable Interest Entity [Line Items]
Maximum Loss Exposure	17,878		20,394
Liability Recognized	0		0
Other tax credit investments | Other Assets
Variable Interest Entity [Line Items]
Maximum loss exposure, current investments	18,000		18,000
Small issuer trust preferred holdings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	276,825		332,455
Liability Recognized	0		0
On-balance sheet trust preferred securitization
Variable Interest Entity [Line Items]
Maximum Loss Exposure	48,306		48,817
Liability Recognized	65,868		65,357
Loans, net of unearned income	112,500		112,500
Trading securities	1,700		1,700
Term borrowings	65,900		65,400
Proprietary residential mortgage securitizations
Variable Interest Entity [Line Items]
Maximum Loss Exposure	1,592		2,151
Liability Recognized	0		0
Holdings of agency mortgage-backed securities
Variable Interest Entity [Line Items]
Maximum Loss Exposure	5,209,810		5,349,287
Liability Recognized	0		0
Trading securities	800,000		500,000
Securities available-for-sale	4,400,000		4,800,000
Commercial loan troubled debt restructurings
Variable Interest Entity [Line Items]
Maximum Loss Exposure	41,926		19,411
Liability Recognized	0		0
Maximum loss exposure, contractual funding commitments	600		300
Loans, net of unearned income	41,400		19,100
Sale-leaseback transaction
Variable Interest Entity [Line Items]
Maximum Loss Exposure	16,327		14,827
Liability Recognized	0		0
Proprietary trust preferred issuances
Variable Interest Entity [Line Items]
Maximum Loss Exposure	0		0
Liability Recognized	$ 202,068		$ 212,378

[1] September 30, 2018 and December 31, 2017 include $20.8 million and $22.7 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.