Variable Interest Entities - Schedule of VIEs Consolidated by FHN (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Assets
Other assets	$ 3,918	$ 3,996
Liabilities:
Other liabilities	2,162	2,315
Rabbi Trusts used for Deferred Compensation Plans
Assets
Other assets	204	195
Liabilities:
Other liabilities	$ 179	$ 165