| Document and Entity Information | Apr. 07, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | CENTURY BANCORP INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000812348 |
| Document Type | 8-K |
| Document Period End Date | Apr. 7, 2021 |
| Entity Incorporation State Country Code | MA |
| Entity File Number | 0-15752 |
| Entity Tax Identification Number | 04-2498617 |
| Entity Address, Address Line One | 400 Mystic Avenue |
| Entity Address, City or Town | Medford |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02155 |
| City Area Code | (781) |
| Local Phone Number | 391-4000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Class A Common Stock, $1.00 par value |
| Trading Symbol | CNBKA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
