Components of Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 9,291	$ 8,547	$ 8,494
Net unrealized gains (losses)		(66)	121	(1,103)
Amounts reclassified from AOCI		126	59	23
Other comprehensive income (loss)		60	180	(1,080)
Ending balance		9,111	9,291	8,547
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(1,188)	(1,368)	(288)
Other comprehensive income (loss)	[1]	60	180	(1,080)
Ending balance	[1]	(1,128)	(1,188)	(1,368)
Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(836)	(973)	(36)
Net unrealized gains (losses)		(15)	137	(937)
Amounts reclassified from AOCI		69	0	0
Other comprehensive income (loss)		54	137	(937)
Ending balance		(782)	(836)	(973)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(80)	(127)	2
Net unrealized gains (losses)		(63)	(5)	(144)
Amounts reclassified from AOCI		49	52	15
Other comprehensive income (loss)		(14)	47	(129)
Ending balance		(94)	(80)	(127)
Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(272)	(268)	(254)
Net unrealized gains (losses)		12	(11)	(22)
Amounts reclassified from AOCI		8	7	8
Other comprehensive income (loss)		20	(4)	(14)
Ending balance		$ (252)	$ (272)	$ (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.