Regulatory Capital and Restrictions - Schedule of Actual Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
First Horizon National Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual Amount	$ 4,154,885	$ 3,940,117
Total Capital, Actual Ratio	11.22%	11.94%
Tier 1 Capital, Actual Amount	$ 3,760,450	$ 3,565,373
Tier 1 Capital, Actual Ratio	10.15%	10.80%
Common Equity Tier 1 Capital, Actual Amount	$ 3,408,936	$ 3,223,702
Common Equity Tier 1 Capital, Actual Ratio	9.20%	9.77%
Leverage, Actual Amount	$ 3,760,450	$ 3,565,373
Leverage, Actual Ratio	9.04%	9.09%
Total Capital, Capital Adequacy purposes, Amount	$ 2,963,663	$ 2,640,208
Total Capital, Capital Adequacy Purposes, Ratio	8.00%	8.00%
Tier 1 Capital, Capital Adequacy Purposes, Amount	$ 2,222,747	$ 1,980,156
Tier 1 Capital, Capital Adequacy Purposes, Ratio	6.00%	6.00%
Common Equity Tier 1 Capital, Capital Adequacy Purposes, Amount	$ 1,667,060	$ 1,485,117
Common Equity Tier 1 Capital, Capital Adequacy Purposes, Ratio	4.50%	4.50%
Leverage, Capital Adequacy Purposes, Amount	$ 1,663,338	$ 1,568,870
Leverage, Capital Adequacy purposes, Ratio	4.00%	4.00%
First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual Amount	$ 3,944,613	$ 3,689,180
Total Capital, Actual Ratio	10.77%	11.32%
Tier 1 Capital, Actual Amount	$ 3,728,683	$ 3,492,541
Tier 1 Capital, Actual Ratio	10.18%	10.72%
Common Equity Tier 1 Capital, Actual Amount	$ 3,433,867	$ 3,197,725
Common Equity Tier 1 Capital, Actual Ratio	9.38%	9.81%
Leverage, Actual Amount	$ 3,728,683	$ 3,492,541
Leverage, Actual Ratio	9.12%	9.10%
Total Capital, Capital Adequacy purposes, Amount	$ 2,930,159	$ 2,607,406
Total Capital, Capital Adequacy Purposes, Ratio	8.00%	8.00%
Tier 1 Capital, Capital Adequacy Purposes, Amount	$ 2,197,620	$ 1,955,555
Tier 1 Capital, Capital Adequacy Purposes, Ratio	6.00%	6.00%
Common Equity Tier 1 Capital, Capital Adequacy Purposes, Amount	$ 1,648,215	$ 1,466,666
Common Equity Tier 1 Capital, Capital Adequacy Purposes, Ratio	4.50%	4.50%
Leverage, Capital Adequacy Purposes, Amount	$ 1,634,695	$ 1,535,279
Leverage, Capital Adequacy purposes, Ratio	4.00%	4.00%
Total Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 3,662,699	$ 3,259,258
Total Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 2,930,159	$ 2,607,406
Tier 1 Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier 1 Capital. To Be Well Capitalized Under Prompt Corrective Action, Amount	$ 2,380,755	$ 2,118,518
Common Equity Tier 1 Capital. To Be Well Capitalized Under Prompt Corrective Action, Ratio	6.50%	6.50%
Leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 2,043,368	$ 1,919,099
Leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%