Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Carrying amount of equity investments without a readily determinable fair value	$ 16.4	$ 16.4	$ 16.3
Unrealized gain for equity investments with readily determinable fair values	$ 0.7	$ 1.1

Equity Securities without Readily Determinable Fair Value, Amount

No definition available.

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