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 Thousands	Sep. 30, 2019	Dec. 31, 2018
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 697,214	$ 386,443
Gross amounts offset in the Statements of Condition	0	0
Net amounts of assets presented in the Statements of Condition	697,214	386,443
Offsetting securities sold under agreements to repurchase	(449)	(261)
Securities collateral (not recognized on FHN’s Statements of Condition)	(693,737)	(382,756)
Net amount	$ 3,028	$ 3,426