Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 10,090
Securities available for sale at fair value	8,954	$ 8,836
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,362	1,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(148)	(162)
Fair Value	1,214	1,209
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,416	5,457
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(622)	(695)
Securities available for sale at fair value	4,796	4,763
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	889	897
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(95)	(109)
Fair Value	794	788
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,650	2,682
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(336)	(369)
Securities available for sale at fair value	2,314	2,313
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	473	474
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53)	(53)
Fair Value	420	421
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,374	1,325
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(141)	(162)
Securities available for sale at fair value	1,239	1,163
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	650	658
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(47)	(62)
Securities available for sale at fair value	605	597
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,090	10,122
Gross Unrealized Gains	10	2
Gross Unrealized Losses	(1,146)	(1,288)
Securities available for sale at fair value	8,954	8,836
Securities available-for-sale, excluding interest only strip \| Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,900	$ 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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