Investment Securities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Marketable Securities [Abstract]
Carrying amount of equity investments without a readily determinable fair value	$ 16.1	$ 16.3
Unrealized gain for equity investments with readily determinable fair values	$ 0.3

Equity Securities without Readily Determinable Fair Value, Amount

No definition available.

Name:	fhn_EquitySecuritieswithoutReadilyDeterminableFairValueAmount
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant