Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,926	$ 8,930
Gross Unrealized Gains	12	1
Gross Unrealized Losses	(821)	(1,035)
Fair Value	8,117	7,896
Amortized Cost	1,245	1,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(162)	(187)
Fair Value	1,083	1,083
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,000	6,900
Amortized Cost	1,200	1,300
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,139	4,223
Gross Unrealized Gains	6	1
Gross Unrealized Losses	(399)	(522)
Fair Value	3,746	3,702
Amortized Cost	782	804
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(92)	(109)
Fair Value	690	695
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,188	3,079
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(260)	(312)
Fair Value	2,932	2,767
Amortized Cost	463	466
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(70)	(78)
Fair Value	393	388
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,223	1,234
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(123)	(161)
Fair Value	1,102	1,073
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	376	394
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39)	(40)
Fair Value	$ 337	$ 354