Master Netting and Similar Agreements – Repurchase, Reverse Repurchase, and Securities Borrowing Transactions - Securities Purchased under Agreements to Resell and Collateral Pledged by Counterparties (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
SECURITIES PURCHASED UNDER AGREEMENTS TO RESELL
Gross amounts of recognized assets	$ 746	$ 613
Gross amounts offset in the Balance Sheets	0	0
Net amounts of assets presented in the Balance Sheets	746	613
Offsetting securities sold under agreements to repurchase	(18)	(5)
Securities collateral (not recognized on FHN’s Balance Sheets)	(724)	(604)
Net amount	$ 4	$ 4