Variable Interest Entities - Schedule of VIEs Consolidated by FHN (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets
Other assets	$ 3,882	$ 4,169
Liabilities:
Other liabilities	2,190	2,382
Rabbi Trusts used for Deferred Compensation Plans
Assets
Other assets	193	177
Liabilities:
Other liabilities	$ 167	$ 150