Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026		Mar. 31, 2025
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 9,142		$ 9,111
Other comprehensive income (loss)	(22)		145
Ending balance	9,465		9,044
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(810)	[1]	(1,128)	[2]
Net unrealized gains (losses)	(30)		134
Amounts reclassified from AOCI	8		11
Other comprehensive income (loss)	(22)	[1]	145	[2]
Ending balance	(832)	[1]	(983)	[2]
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(512)		(782)
Net unrealized gains (losses)	(14)		116
Amounts reclassified from AOCI	0		0
Other comprehensive income (loss)	(14)		116
Ending balance	(526)		(666)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(42)		(94)
Net unrealized gains (losses)	(16)		18
Amounts reclassified from AOCI	6		9
Other comprehensive income (loss)	(10)		27
Ending balance	(52)		(67)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(256)		(252)
Net unrealized gains (losses)	0		0
Amounts reclassified from AOCI	2		2
Other comprehensive income (loss)	2		2
Ending balance	$ (254)		$ (250)

[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.