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	Jun. 30, 2022	Dec. 31, 2021
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 423	$ 488
Gross amounts offset in the Balance Sheets	0	0
Net amounts of assets presented in the Balance Sheets	423	488
Offsetting securities sold under agreements to repurchase	(6)	(10)
Securities collateral (not recognized on FHN’s Balance Sheets)	(415)	(476)
Net amount	$ 2	$ 2