Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,838	$ 8,930
Gross Unrealized Gains	14	1
Gross Unrealized Losses	(750)	(1,035)
Fair Value	8,102	7,896
Amortized Cost	1,229	1,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(150)	(187)
Fair Value	1,079	1,083
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,000	6,900
Amortized Cost	900	1,300
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,039	4,223
Gross Unrealized Gains	7	1
Gross Unrealized Losses	(364)	(522)
Fair Value	3,682	3,702
Amortized Cost	769	804
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(81)	(109)
Fair Value	688	695
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,078	3,079
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(239)	(312)
Fair Value	2,843	2,767
Amortized Cost	460	466
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(69)	(78)
Fair Value	391	388
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,355	1,234
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(117)	(161)
Fair Value	1,240	1,073
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	366	394
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(30)	(40)
Fair Value	$ 337	$ 354

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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