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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% |  |

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