Variable Interest Entities - Summary Of VIE Consolidated By FHN (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets
Other assets	$ 3,696	$ 4,072
Total assets	87,513	84,209
Liabilities:
Other liabilities	1,712	1,699
Total liabilities	79,206	75,902
Rabbi Trusts used for Deferred Compensation Plans
Assets
Other assets	200	195
Total assets	200	195
Liabilities:
Other liabilities	173	165
Total liabilities	$ 173	$ 165