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	Sep. 30, 2025	Dec. 31, 2024
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 679	$ 572
Gross amounts offset in the Balance Sheets	0	0
Net amounts of assets presented in the Balance Sheets	679	572
Offsetting securities sold under agreements to repurchase	0	0
Securities collateral (not recognized on FHN’s Balance Sheets)	(673)	(567)
Net amount	$ 6	$ 5