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, 2020	Dec. 31, 2019
Securities Purchased under Agreements to Resell [Abstract]
Gross amounts of recognized assets	$ 302,267	$ 586,629
Gross amounts offset in the Statements of Condition	0	0
Net amounts of assets presented in the Statements of Condition	302,267	586,629
Offsetting securities sold under agreements to repurchase	(23,207)	(21,004)
Securities collateral (not recognized on FHN’s Statements of Condition)	(277,660)	(562,702)
Net amount	$ 1,400	$ 2,923