Components of Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 9,111	$ 9,291	$ 8,547
Other comprehensive income		318	60	180
Ending balance		9,142	9,111	9,291
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(1,128)	(1,188)	(1,368)
Net unrealized gains (losses)		275	(66)	121
Amounts reclassified from AOCI		43	126	59
Other comprehensive income	[1]	318	60	180
Ending balance	[1]	(810)	(1,128)	(1,188)
Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(782)	(836)	(973)
Net unrealized gains (losses)		270	(15)	137
Amounts reclassified from AOCI		0	69	0
Other comprehensive income		270	54	137
Ending balance		(512)	(782)	(836)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(94)	(80)	(127)
Net unrealized gains (losses)		17	(63)	(5)
Amounts reclassified from AOCI		35	49	52
Other comprehensive income		52	(14)	47
Ending balance		(42)	(94)	(80)
Pension and Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(252)	(272)	(268)
Net unrealized gains (losses)		(12)	12	(11)
Amounts reclassified from AOCI		8	8	7
Other comprehensive income		(4)	20	(4)
Ending balance		$ (256)	$ (252)	$ (272)

[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.