Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 9,496
Securities available for sale at fair value	8,391	$ 8,836
Amortized Cost	1,323	1,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(162)	(162)
Fair Value	1,161	1,209
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,061	5,457
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(579)	(695)
Securities available for sale at fair value	4,484	4,763
Amortized Cost	471	897
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(66)	(109)
Fair Value	405	788
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,487	2,682
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(341)	(369)
Securities available for sale at fair value	2,146	2,313
Amortized Cost	852	474
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(96)	(53)
Fair Value	756	421
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,321	1,325
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(151)	(162)
Securities available for sale at fair value	1,172	1,163
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	627	658
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(41)	(62)
Securities available for sale at fair value	589	597
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,496	10,122
Gross Unrealized Gains	7	2
Gross Unrealized Losses	(1,112)	(1,288)
Securities available for sale at fair value	8,391	8,836
Securities available-for-sale, excluding interest only strip \| Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,900	$ 6,500

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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