| Document And Entity Information | May 21, 2020 |
| Document Information Line Items | |
| Entity Registrant Name | First Horizon National Corporation |
| Document Type | 8-K |
| Current Fiscal Year End Date | --12-31 |
| Amendment Flag | false |
| Entity Central Index Key | 0000036966 |
| Document Period End Date | May 21, 2020 |
| Entity Emerging Growth Company | false |
| Entity Incorporation, State or Country Code | TN |
| 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 |
| City Area Code | 901 |
| Local Phone Number | 523-4444 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Common Class A [Member] | |
| Document Information Line Items | |
| Trading Symbol | FHN |
| Title of 12(b) Security | $0.625 Par Value Common Capital Stock |
| Security Exchange Name | NYSE |
| Series A Preferred Stock [Member] | |
| Document Information Line Items | |
| Trading Symbol | FHN PR A |
| Title of 12(b) Security | Depositary Shares, each representing a 1/4,000th interest in a share of Non-Cumulative Perpetual Preferred Stock, Series A |
| Security Exchange Name | NYSE |
