Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended												9 Months Ended
	Sep. 30, 2023		Jun. 30, 2023		Mar. 31, 2023		Sep. 30, 2022		Jun. 30, 2022		Mar. 31, 2022		Sep. 30, 2023		Sep. 30, 2022
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 8,960		$ 8,895		$ 8,547		$ 8,551		$ 8,697		$ 8,494		$ 8,547		$ 8,494
Net unrealized gains (losses)	(239)						(471)						(258)		(1,152)
Amounts reclassified from AOCI	16						7						44		13
Other comprehensive income (loss)	(223)		(151)		160		(464)		(252)		(423)		(214)		(1,139)
Balance, end of period	8,793		8,960		8,895		8,283		8,551		8,697		8,793		8,283
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(1,359)	[1]	(1,208)	[1]	(1,368)	[1]	(963)	[2]	(711)	[2]	(288)	[2]	(1,368)	[1]	(288)	[2]
Other comprehensive income (loss)	(223)	[1]	(151)	[1]	160	[1]	(464)	[2]	(252)	[2]	(423)	[2]
Balance, end of period	(1,582)	[1]	(1,359)	[1]	(1,208)	[1]	(1,427)	[2]	(963)	[2]	(711)	[2]	(1,582)	[1]	(1,427)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(964)				(973)		(671)				(36)		(973)		(36)
Net unrealized gains (losses)	(206)						(368)						(197)		(1,003)
Amounts reclassified from AOCI	0						0						0		0
Other comprehensive income (loss)	(206)						(368)						(197)		(1,003)
Balance, end of period	(1,170)		(964)				(1,039)		(671)				(1,170)		(1,039)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(129)				(127)		(40)				3		(127)		3
Net unrealized gains (losses)	(34)						(103)						(60)		(149)
Amounts reclassified from AOCI	14						4						38		7
Other comprehensive income (loss)	(20)						(99)						(22)		(142)
Balance, end of period	(149)		(129)				(139)		(40)				(149)		(139)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(266)				$ (268)		(252)				$ (255)		(268)		(255)
Net unrealized gains (losses)	1						0						(1)		0
Amounts reclassified from AOCI	2						3						6		6
Other comprehensive income (loss)	3						3						5		6
Balance, end of period	$ (263)		$ (266)				$ (249)		$ (252)				$ (263)		$ (249)

[1] Due to the nature of the preferred stock issued by FHN and its subsidiaries, all components of other comprehensive income (loss) have been attributed solely to FHN as the controlling interest holder.

[2] Due to the nature of the preferred stock issued by FHN and its subsidiaries, all components of other comprehensive income (loss) have been attributed solely to FHN as the controlling interest holder.