Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Feb. 29, 2012 Common Class A [Member]	Feb. 29, 2012 Common Class B [Member]
Entity Registrant Name	CENTURY BANCORP INC
Entity Central Index Key	0000812348
Document Type	10-K
Document Period End Date	Dec. 31, 2011
Amendment Flag	false
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float		$ 94,674,012
Entity Common Stock, Shares Outstanding			3,551,542	1,994,380