Document and Entity Information
| Document and Entity Information | Jul. 27, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001600125 |
| Document Type | 8-K |
| Document Period End Date | Jul. 27, 2021 |
| Entity Registrant Name | MERIDIAN BANCORP, INC. |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 001-36573 |
| Entity Tax Identification Number | 46-5396964 |
| Entity Address, Address Line One | 67 Prospect Street |
| Entity Address, City or Town | Peabody |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01960 |
| City Area Code | (617) |
| Local Phone Number | 567-1500 |
| Written Communications | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, par value $0.01 per share |
| Trading Symbol | EBSB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |