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Document And Entity Information - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Jan. 31, 2020
Jun. 30, 2019
Entity Information [Line Items]      
Entity Incorporation, State or Country Code TN    
Entity Registrant Name FIRST HORIZON NATIONAL CORPORATION    
Entity Central Index Key 0000036966    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   311,602,613  
Document Transition Report false    
Document Annual Report true    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Shell Company false    
Entity Tax Identification Number 62-0803242    
Entity Address, Address Line One 165 Madison Avenue    
Entity Address, City or Town Memphis,    
Entity Address, State or Province TN    
Entity Address, Postal Zip Code 38103    
City Area Code 901    
Local Phone Number 523-4444    
Entity File Number 001-15185    
Entity Public Float     $ 4.6
Documents Incorporated by Reference
Portions of the Proxy Statement to be furnished to shareholders in connection with Annual Meeting of shareholders scheduled for April 28, 2020: Part III of this Report
    
Common Class A      
Entity Information [Line Items]      
Title of 12(b) Security $.625 Par Value Common Capital Stock    
Security Exchange Name NYSE    
Trading Symbol  FHN    
Series A Preferred Stock      
Entity Information [Line Items]      
Title of 12(b) Security Depositary Shares, each representing a 1/4,000th interest ina share of Non-Cumulative Perpetual Preferred Stock, Series A    
Security Exchange Name NYSE    
Trading Symbol FHN PR A