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	Jun. 30, 2018	Dec. 31, 2017
Offsetting [Abstract]
Gross amounts of recognized assets	$ 782,765	$ 725,609
Gross amounts offset in the Statements of Condition	0	0
Net amounts of assets presented in the Statements of Condition	782,765	725,609
Gross amounts not offset in the Statements of Condition
Offsetting securities sold under agreements to repurchase	(2,090)	(259)
Securities collateral (not recognized on FHN’s Statements of Condition)	(772,347)	(720,036)
Net amount	$ 8,328	$ 5,314