Summary of Significant Accounting Policies - ASU 2016-13 (Details) $ in Millions	Jan. 01, 2020 USD ($)	Dec. 31, 2019	Dec. 31, 2018
First Horizon National Corporation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Regulatory capital ratio CET1		9.20%	9.77%
Forecast | Subsequent Event | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings, net of tax	$ 100
Forecast | First Horizon National Corporation | Subsequent Event | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Coverage ratio on total loans	0.0100
Regulatory capital ratio CET1	0.07%