| Document and Entity Information Document | Sep. 15, 2020 | Jun. 29, 2020 |
| Entity Information [Line Items] | ||
| Entity Central Index Key | 0000036966 | |
| Entity Incorporation, State or Country Code | TN | |
| Document Type | 8-K/A | |
| Entity Registrant Name | First Horizon National Corporation | |
| City Area Code | 901 | |
| Local Phone Number | 523-4444 | |
| Entity File Number | 001-15185 | |
| 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 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Document Period End Date | Jun. 29, 2020 | |
| Amendment Flag | false | |
| Common Class A [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | $0.625 Par Value Common Capital Stock | |
| Trading Symbol | FHN | |
| Security Exchange Name | NYSE | |
| Series A Preferred Stock [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares, each representing a 1/4,000th interest in | |
| Trading Symbol | FHN PR A | |
| Security Exchange Name | NYSE | |
| Series B Preferred Stock [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares, each representing a 1/400th interest in | |
| Trading Symbol | FHN PR B | |
| Security Exchange Name | NYSE | |
| Series C Preferred Stock [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares, each representing a 1/400th interest in | |
| Trading Symbol | FHN PR C | |
| Security Exchange Name | NYSE | |
| Series D Preferred Stock [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares, each representing a 1/400th interest in | |
| Trading Symbol | FHN PR D | |
| Security Exchange Name | NYSE | |
| Series E Preferred Stock [Member] | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares, each representing a 1/4,000th interest in | |
| Trading Symbol | FHN PR E | |
| Security Exchange Name | NYSE |
