Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	$ 8,841	$ 8,930
Gross Unrealized Gains	19	1
Gross Unrealized Losses	(695)	(1,035)
Fair Value	8,165	7,896
Amortized Cost	1,216	1,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(143)	(187)
Fair Value	1,073	1,083
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	7,200	6,900
Amortized Cost	1,100	1,200
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	3,964	4,223
Gross Unrealized Gains	9	1
Gross Unrealized Losses	(332)	(522)
Fair Value	3,641	3,702
Amortized Cost	458	804
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(67)	(109)
Fair Value	391	695
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	3,092	3,079
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(229)	(312)
Fair Value	2,869	2,767
Amortized Cost	758	466
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(76)	(78)
Fair Value	682	388
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	1,424	1,234
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(109)	(161)
Fair Value	1,317	1,073
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	361	394
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(25)	(40)
Fair Value	$ 338	$ 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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