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, 2021	Dec. 31, 2020
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 355	$ 380
Gross amounts offset in the Balance Sheets	0	0
Net amounts of assets presented in the Balance Sheets	355	380
Offsetting securities sold under agreements to repurchase	(12)	0
Securities collateral (not recognized on FHN’s Balance Sheets)	(341)	(379)
Net amount	$ 2	$ 1