Components of Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 8,547	$ 8,494		$ 8,307
Net unrealized gains (losses)		121	(1,103)		(149)
Amounts reclassified from AOCI		59	23		1
Other comprehensive income (loss)		180	(1,080)		(148)
Ending balance		9,291	8,547		8,494
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(1,368)	(288)		(140)
Other comprehensive income (loss)		180	(1,080)	[1]	(148)	[1]
Ending balance	[1]	(1,188)	(1,368)		(288)
Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(973)	(36)		108
Net unrealized gains (losses)		137	(937)		(144)
Amounts reclassified from AOCI		0	0		0
Other comprehensive income (loss)		137	(937)		(144)
Ending balance		(836)	(973)		(36)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(127)	2		12
Net unrealized gains (losses)		(5)	(144)		(3)
Amounts reclassified from AOCI		52	15		(7)
Other comprehensive income (loss)		47	(129)		(10)
Ending balance		(80)	(127)		2
Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(268)	(254)		(260)
Net unrealized gains (losses)		(11)	(22)		(2)
Amounts reclassified from AOCI		7	8		8
Other comprehensive income (loss)		(4)	(14)		6
Ending balance		$ (272)	$ (268)		$ (254)

[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.