Parent Company Financial Statements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ASSETS:
Cash	$ 53,646	$ 50,187
Other assets	66,031	63,501
Total assets	3,086,209	2,743,225
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	2,906,219	2,582,576
Subordinated debentures (Note 12)	36,083	36,083
Stockholders' equity	179,990	160,649	145,025	132,730
Total liabilities and stockholders' equity	3,086,209	2,743,225
Century Bancorp, Inc. [Member]
ASSETS:
Cash	19,536	23,467
Investment in subsidiary, at equity	193,499	170,642
Other assets	3,145	2,730
Total assets	216,180	196,839
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	107	107
Subordinated debentures (Note 12)	36,083	36,083
Stockholders' equity	179,990	160,649
Total liabilities and stockholders' equity	$ 216,180	$ 196,839