Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 9,116
Fair Value	7,924	$ 8,391
Amortized Cost	1,297	1,323
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(189)	(162)
Fair Value	1,108	1,161
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,300	1,300
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,852	5,061
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(622)	(579)
Fair Value	4,231	4,484
Amortized Cost	828	852
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(113)	(96)
Fair Value	715	756
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,374	2,487
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(354)	(341)
Fair Value	2,020	2,146
Amortized Cost	469	471
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(76)	(66)
Fair Value	393	405
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,282	1,321
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(164)	(151)
Fair Value	1,118	1,172
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	608	627
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(54)	(41)
Fair Value	555	589
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,116	9,496
Gross Unrealized Gains	2	7
Gross Unrealized Losses	(1,194)	(1,112)
Fair Value	7,924	8,391
Securities available-for-sale, excluding interest only strip \| Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,900	$ 7,600

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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