Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,754
Fair Value	8,707	$ 8,047
Securities pledged as collateral	6,500	6,400
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	712	10
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	0
Fair Value	705	10
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		613
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		613
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,062	3,722
Gross Unrealized Gains	42	92
Gross Unrealized Losses	(49)	(2)
Fair Value	5,055	3,812
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	509
Gross Unrealized Gains	0
Gross Unrealized Losses	(5)
Fair Value	504
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,296	2,380
Gross Unrealized Gains	8	29
Gross Unrealized Losses	(47)	(3)
Fair Value	2,257	2,406
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	203
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	201
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	861	672
Gross Unrealized Gains	4	12
Gross Unrealized Losses	(15)	0
Fair Value	850	684
Corporate and other debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		40
Gross Unrealized Gains		1
Gross Unrealized Losses		(1)
Fair Value		40
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		10
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		10
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	535	445
Gross Unrealized Gains	11	15
Gross Unrealized Losses	(1)	0
Fair Value	545	460
Securities available-for-sale, excluding interest only strip
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,754	7,872
Gross Unrealized Gains	65	149
Gross Unrealized Losses	(112)	(6)
Fair Value	$ 8,707	8,015
Interest-only strips
Debt Securities, Available-for-sale [Line Items]
Fair Value		$ 32

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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