Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Securities available for sale:
Amortized Cost	$ 8,579	$ 8,841
Gross Unrealized Gains	7	19
Gross Unrealized Losses	(713)	(695)
Fair Value	7,873	8,165
Securities held to maturity:
Amortized Cost	1,189	1,216
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(151)	(143)
Fair Value	1,038	1,073
Asset Pledged as Collateral
Securities available for sale:
Amortized Cost	6,800	7,200
Securities held to maturity:
Amortized Cost	789,000	1,100
Government agency issued MBS
Securities available for sale:
Amortized Cost	3,727	3,964
Gross Unrealized Gains	3	9
Gross Unrealized Losses	(340)	(332)
Fair Value	3,390	3,641
Securities held to maturity:
Amortized Cost	735	758
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(80)	(76)
Fair Value	655	682
Government agency issued CMO
Securities available for sale:
Amortized Cost	2,942	3,092
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(238)	(229)
Fair Value	2,706	2,869
Securities held to maturity:
Amortized Cost	454	458
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(71)	(67)
Fair Value	383	391
Other U.S. government agencies
Securities available for sale:
Amortized Cost	1,560	1,424
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(110)	(109)
Fair Value	1,451	1,317
States and municipalities
Securities available for sale:
Amortized Cost	350	361
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(25)	(25)
Fair Value	$ 326	$ 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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