Variable Interest Entities - Summary of VIE Consolidated By FHN (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Other assets	$ 4,072	$ 2,297
Total assets	84,209	43,311
Liabilities:
Other liabilities	1,699	990
Total liabilities	75,902	38,235
Rabbi Trusts Used For Deferred Compensation Plans
Assets:
Other assets	164	92
Total assets	164	92
Liabilities:
Other liabilities	142	71
Total liabilities	$ 142	$ 71