Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended												9 Months Ended
	Sep. 30, 2024		Jun. 30, 2024		Mar. 31, 2024		Sep. 30, 2023		Jun. 30, 2023		Mar. 31, 2023		Sep. 30, 2024		Sep. 30, 2023
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 8,955		$ 9,173		$ 9,291		$ 8,960		$ 8,895		$ 8,547		$ 9,291		$ 8,547
Other comprehensive income (loss)	292		(10)		(83)		(223)		(151)		160		199		(214)
Balance, ending of period	9,316		8,955		9,173		8,793		8,960		8,895		9,316		8,793
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(1,281)	[1]	(1,271)	[1]	(1,188)	[1]	(1,359)	[2]	(1,208)	[2]	(1,368)	[2]	(1,188)	[1]	(1,368)	[2]
Net unrealized gains (losses)	277						(239)						154		(258)
Amounts reclassified from AOCI	15						16						45		44
Other comprehensive income (loss)	292	[1]	(10)	[1]	(83)	[1]	(223)	[2]	(151)	[2]	160	[2]	199		(214)
Balance, ending of period	(989)	[1]	(1,281)	[1]	(1,271)	[1]	(1,582)	[2]	(1,359)	[2]	(1,208)	[2]	(989)	[1]	(1,582)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(901)				(836)		(964)				(973)		(836)		(973)
Net unrealized gains (losses)	232						(206)						167		(197)
Amounts reclassified from AOCI	0						0						0		0
Other comprehensive income (loss)	232						(206)						167		(197)
Balance, ending of period	(669)		(901)				(1,170)		(964)				(669)		(1,170)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(112)				(80)		(129)				(127)		(80)		(127)
Net unrealized gains (losses)	45						(34)						(13)		(60)
Amounts reclassified from AOCI	13						14						39		38
Other comprehensive income (loss)	58						(20)						26		(22)
Balance, ending of period	(54)		(112)				(149)		(129)				(54)		(149)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(268)				$ (272)		(266)				$ (268)		(272)		(268)
Net unrealized gains (losses)	0						1						0		(1)
Amounts reclassified from AOCI	2						2						6		6
Other comprehensive income (loss)	2						3						6		5
Balance, ending of period	$ (266)		$ (268)				$ (263)		$ (266)				$ (266)		$ (263)

[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.