Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended
	Jun. 30, 2025		Mar. 31, 2025		Jun. 30, 2024		Mar. 31, 2024		Jun. 30, 2025		Jun. 30, 2024
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 9,044		$ 9,111		$ 9,173		$ 9,291		$ 9,111		$ 9,291
Other comprehensive income (loss)	71		145		(10)		(83)		216		(93)
Balance, ending of period	9,257		9,044		8,955		9,173		9,257		8,955
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(983)	[1]	(1,128)	[1]	(1,271)	[2]	(1,188)	[2]	(1,128)	[1]	(1,188)	[2]
Net unrealized gains (losses)	60				(25)				194		(123)
Amounts reclassified from AOCI	11				15				22		30
Other comprehensive income (loss)	71	[1]	145	[1]	(10)	[2]	(83)	[2]	216		(93)
Balance, ending of period	(912)	[1]	(983)	[1]	(1,281)	[2]	(1,271)	[2]	(912)	[1]	(1,281)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(666)		(782)		(891)		(836)		(782)		(836)
Net unrealized gains (losses)	53				(10)				169		(65)
Amounts reclassified from AOCI	0				0				0		0
Other comprehensive income (loss)	53				(10)				169		(65)
Balance, ending of period	(613)		(666)		(901)		(891)		(613)		(901)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(67)		(94)		(110)		(80)		(94)		(80)
Net unrealized gains (losses)	7				(15)				25		(58)
Amounts reclassified from AOCI	9				13				18		26
Other comprehensive income (loss)	16				(2)				43		(32)
Balance, ending of period	(51)		(67)		(112)		(110)		(51)		(112)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(250)		(252)		(270)		(272)		(252)		(272)
Net unrealized gains (losses)	0				0				0		0
Amounts reclassified from AOCI	2				2				4		4
Other comprehensive income (loss)	2				2				4		4
Balance, ending of period	$ (248)		$ (250)		$ (268)		$ (270)		$ (248)		$ (268)

[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.