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, 2023	Dec. 31, 2022
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 168	$ 353
Gross amounts offset in the Balance Sheets	0	0
Net amounts of assets presented in the Balance Sheets	168	353
Offsetting securities sold under agreements to repurchase	0	(10)
Securities collateral (not recognized on FHN’s Balance Sheets)	(166)	(340)
Net amount	$ 2	$ 3