| Document and Entity Information | Feb. 16, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 16, 2021 |
| Entity Registrant Name | Randolph Bancorp, Inc. |
| Entity Central Index Key | 0001667161 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-37780 |
| Entity Incorporation, State or Country Code | MA |
| Entity Tax Identification Number | 81-1844402 |
| Entity Address, Address Line One | 10 Cabot Place |
| Entity Address, City or Town | Stoughton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02072 |
| City Area Code | 781 |
| Local Phone Number | 963-2100 |
| 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 $.01 per share |
| Trading Symbol | RNDB |
| Security Exchange Name | NASDAQ |
