Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ASSETS:
Cash	$ 59,956,000	$ 53,646,000
Other assets	87,897,000	66,031,000
Total assets	3,431,154,000	3,086,209,000
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	3,254,682,000	2,906,219,000
Subordinated debentures	36,083,000	36,083,000
Stockholders' equity	176,472,000	179,990,000	160,649,000	145,025,000
Total liabilities and stockholders' equity	3,431,154,000	3,086,209,000
Century Bancorp, Inc. [Member]
ASSETS:
Cash	12,245,000	19,536,000
Investment in subsidiary, at equity	193,783,000	193,499,000
Other assets	6,634,000	3,145,000
Total assets	212,662,000	216,180,000
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	107,000	107,000
Subordinated debentures	36,083,000	36,083,000
Stockholders' equity	176,472,000	179,990,000
Total liabilities and stockholders' equity	$ 212,662,000	$ 216,180,000