Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024		Mar. 31, 2023
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 9,291		$ 8,547
Net unrealized gains (losses)	(98)		147
Amounts reclassified from AOCI	15		13
Other comprehensive income (loss)	(83)		160
Balance, end of period	9,173		8,895
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(1,188)	[1]	(1,368)	[2]
Other comprehensive income (loss)	(83)	[1]	160	[2]
Balance, end of period	(1,271)	[1]	(1,208)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(836)		(973)
Net unrealized gains (losses)	(55)		114
Amounts reclassified from AOCI	0		0
Other comprehensive income (loss)	(55)		114
Balance, end of period	(891)		(859)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(80)		(127)
Net unrealized gains (losses)	(43)		33
Amounts reclassified from AOCI	13		11
Other comprehensive income (loss)	(30)		44
Balance, end of period	(110)		(83)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(272)		(268)
Net unrealized gains (losses)	0		0
Amounts reclassified from AOCI	2		2
Other comprehensive income (loss)	2		2
Balance, end of period	$ (270)		$ (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.