| Cover | Oct. 19, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 19, 2020 |
| Entity File Number | 000-06253 |
| Entity Registrant Name | SIMMONS FIRST NATIONAL CORPORATION |
| Entity Central Index Key | 0000090498 |
| Entity Tax Identification Number | 71-0407808 |
| Entity Incorporation, State or Country Code | AR |
| Entity Address, Address Line One | 501 Main Street |
| Entity Address, City or Town | Pine Bluff |
| Entity Address, State or Province | AR |
| Entity Address, Postal Zip Code | 71601 |
| City Area Code | (870) |
| Local Phone Number | 541-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | SFNC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
