Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended												9 Months Ended
	Sep. 30, 2025		Jun. 30, 2025		Mar. 31, 2025		Sep. 30, 2024		Jun. 30, 2024		Mar. 31, 2024		Sep. 30, 2025		Sep. 30, 2024
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 9,257		$ 9,044		$ 9,111		$ 8,955		$ 9,173		$ 9,291		$ 9,111		$ 9,291
Other comprehensive income	63		71		145		292		(10)		(83)		279		199
Balance, ending of period	9,244		9,257		9,044		9,316		8,955		9,173		9,244		9,316
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(912)	[1]	(983)	[1]	(1,128)	[1]	(1,281)	[2]	(1,271)	[2]	(1,188)	[2]	(1,128)	[1]	(1,188)	[2]
Net unrealized gains (losses)	52						277						246		154
Amounts reclassified from AOCI	11						15						33		45
Other comprehensive income	63	[1]	71	[1]	145	[1]	292	[2]	(10)	[2]	(83)	[2]	279		199
Balance, ending of period	(849)	[1]	(912)	[1]	(983)	[1]	(989)	[2]	(1,281)	[2]	(1,271)	[2]	(849)	[1]	(989)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(613)				(782)		(901)				(836)		(782)		(836)
Net unrealized gains (losses)	56						232						225		167
Amounts reclassified from AOCI	0						0						0		0
Other comprehensive income	56						232						225		167
Balance, ending of period	(557)		(613)				(669)		(901)				(557)		(669)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(51)				(94)		(112)				(80)		(94)		(80)
Net unrealized gains (losses)	(4)						45						21		(13)
Amounts reclassified from AOCI	9						13						27		39
Other comprehensive income	5						58						48		26
Balance, ending of period	(46)		(51)				(54)		(112)				(46)		(54)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(248)				$ (252)		(268)				$ (272)		(252)		(272)
Net unrealized gains (losses)	0						0						0		0
Amounts reclassified from AOCI	2						2						6		6
Other comprehensive income	2						2						6		6
Balance, ending of period	$ (246)		$ (248)				$ (266)		$ (268)				$ (246)		$ (266)

[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.