Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Jul. 31, 2021
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Registrant Name	CENTURY BANCORP, INC.
Entity Central Index Key	0000812348
Entity Filer Category	Accelerated Filer
Entity Tax Identification Number	04-2498617
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Trading Symbol	CNBKA
Entity Address, State or Province	MA
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	0-15752
Entity Incorporation, State or Country Code	MA
Entity Address, City or Town	MEDFORD
Entity Address, Address Line One	400 MYSTIC AVENUE
Entity Address, Postal Zip Code	02155
City Area Code	781
Local Phone Number	391-4000
Title of 12(b) Security	Class A Common Stock
Security Exchange Name	NASDAQ
Class A Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,661,569
Class B Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,906,340