Variable Interest Entities - Schedule of VIEs Consolidated by FHN (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Assets
Other assets	$ 3,856	$ 3,888
Liabilities:
Other liabilities	2,033	2,076
Rabbi Trusts used for Deferred Compensation Plans
Assets
Other assets	196	202
Liabilities:
Other liabilities	$ 178	$ 176