Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Securities available for sale:
Amortized Cost	$ 8,841	$ 8,841
Gross Unrealized Gains	12	19
Gross Unrealized Losses	(707)	(695)
Fair Value	8,146	8,165
Securities held to maturity:
Amortized Cost	1,204	1,216
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(148)	(143)
Fair Value	1,056	1,073
Asset Pledged as Collateral
Securities available for sale:
Amortized Cost	7,300	7,200
Securities held to maturity:
Amortized Cost	1,100	1,100
Government agency issued MBS
Securities available for sale:
Amortized Cost	3,852	3,964
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(331)	(332)
Fair Value	3,525	3,641
Securities held to maturity:
Amortized Cost	749	758
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(78)	(76)
Fair Value	671	682
Government agency issued CMO
Securities available for sale:
Amortized Cost	3,026	3,092
Gross Unrealized Gains	4	6
Gross Unrealized Losses	(238)	(229)
Fair Value	2,792	2,869
Securities held to maturity:
Amortized Cost	455	458
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(70)	(67)
Fair Value	385	391
Other U.S. government agencies
Securities available for sale:
Amortized Cost	1,603	1,424
Gross Unrealized Gains	3	2
Gross Unrealized Losses	(106)	(109)
Fair Value	1,500	1,317
States and municipalities
Securities available for sale:
Amortized Cost	360	361
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(32)	(25)
Fair Value	$ 329	$ 338

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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