| Document and Entity Information | Jul. 14, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 14, 2021 |
| Entity Registrant Name | Reliant Bancorp, Inc. |
| Entity Incorporation, State or Country Code | TN |
| Entity File Number | 001-37391 |
| Entity Tax Identification Number | 37-1641316 |
| Entity Address, Address Line One | 1736 Carothers Parkway |
| Entity Address, Address Line Two | Suite 100 |
| Entity Address, City or Town | Brentwood |
| Entity Address, State or Province | TN |
| Entity Address, Postal Zip Code | 37027 |
| City Area Code | 615 |
| Local Phone Number | 221-2020 |
| Written Communications | true |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001606440 |
| Title of 12(b) Security | Common Stock, $1.00 par value per share |
| Trading Symbol | RBNC |
| Security Exchange Name | NASDAQ |
