Variable Interest Entities - Summary of VIE Consolidated By FHN (Details) - USD ($) $ in Thousands	Sep. 30, 2018		Jun. 30, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets:
Cash and due from banks	$ 642,051			$ 639,073
Loans, net of unearned income	27,350,214	[1]		27,658,929	[1]	$ 20,166,091
Less: Allowance for loan losses	185,959		$ 185,462	189,555		$ 194,867	$ 197,257	$ 202,068
Total net loans	27,164,255			27,469,374
Other assets	1,883,077			1,723,189
Total assets	40,635,924			41,423,388
Liabilities:
Term borrowings	1,200,134			1,218,097
Other liabilities	552,921			549,234
Total liabilities	35,893,993			36,842,900
On-Balance Sheet Consumer Loan Securitization
Assets:
Cash and due from banks	0			0
Loans, net of unearned income	17,106			24,175
Less: Allowance for loan losses	0			0
Total net loans	17,106			24,175
Other assets	35			47
Total assets	17,141			24,222
Liabilities:
Term borrowings	4,071			11,226
Other liabilities	0			2
Total liabilities	4,071			11,228
Rabbi Trusts Used for Deferred Compensation Plans
Assets:
Other assets	84,170			80,479
Total assets	84,170			80,479
Liabilities:
Other liabilities	63,017			61,733
Total liabilities	$ 63,017			$ 61,733

[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.