Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 10,122
Fair Value	8,836	$ 8,707
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,371	712
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(162)	(7)
Fair Value	1,209	705
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,457	5,062
Gross Unrealized Gains	1	42
Gross Unrealized Losses	(695)	(49)
Fair Value	4,763	5,055
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	474	509
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53)	(5)
Fair Value	421	504
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,682	2,296
Gross Unrealized Gains	0	8
Gross Unrealized Losses	(369)	(47)
Fair Value	2,313	2,257
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	897	203
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(109)	(2)
Fair Value	788	201
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,325	861
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(162)	(15)
Fair Value	1,163	850
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	658	535
Gross Unrealized Gains	1	11
Gross Unrealized Losses	(62)	(1)
Fair Value	597	545
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,122	8,754
Gross Unrealized Gains	2	65
Gross Unrealized Losses	(1,288)	(112)
Fair Value	8,836	8,707
Securities available-for-sale, excluding interest only strip \| Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,500	$ 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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