Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 9,648
Securities available for sale at fair value	8,100	$ 8,836
Securities held to maturity (fair value of $1,104 and $1,209, respectively)	1,335	1,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(231)	(162)
Fair Value	1,104	1,209
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,156	5,457
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(831)	(695)
Securities available for sale at fair value	4,325	4,763
Securities held to maturity (fair value of $1,104 and $1,209, respectively)	863	897
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(143)	(109)
Fair Value	720	788
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,528	2,682
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(428)	(369)
Securities available for sale at fair value	2,100	2,313
Securities held to maturity (fair value of $1,104 and $1,209, respectively)	472	474
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(88)	(53)
Fair Value	384	421
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,334	1,325
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(206)	(162)
Securities available for sale at fair value	1,128	1,163
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	630	658
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(83)	(62)
Securities available for sale at fair value	547	597
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,648	10,122
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(1,548)	(1,288)
Securities available for sale at fair value	8,100	8,836
Securities available-for-sale, excluding interest only strip \| Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,400	$ 6,500

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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