Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Cash	$ 62,400	$ 52,877
Other assets	116,360	110,582
Total assets	4,462,608	3,947,441
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	4,222,567	3,732,897
Subordinated debentures	36,083	36,083
Stockholders' equity	240,041	214,544	$ 192,500	$ 176,472
Total liabilities and stockholders' equity	4,462,608	3,947,441
Century Bancorp, Inc. [Member]
ASSETS
Cash	2,768	5,230
Investment in subsidiary, at equity	263,070	236,629
Other assets	10,335	8,808
Total assets	276,173	250,667
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	49	40
Subordinated debentures	36,083	36,083
Stockholders' equity	240,041	214,544
Total liabilities and stockholders' equity	$ 276,173	$ 250,667