Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS:
Cash	$ 44,420	$ 89,540
Other assets	157,354	123,094
Total assets	5,492,424	5,163,935
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	5,159,843	4,863,496
Subordinated debentures	36,083	36,083
Stockholders' equity	332,581	300,439	$ 260,297	$ 240,041
Total liabilities and stockholders' equity	5,492,424	5,163,935
Century Bancorp, Inc. [Member]
ASSETS:
Cash	3,177	1,263
Investment in subsidiary, at equity	353,489	322,775
Other assets	16,325	16,991
Total assets	372,991	341,029
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	4,327	4,507
Subordinated debentures	36,083	36,083
Stockholders' equity	332,581	300,439
Total liabilities and stockholders' equity	$ 372,991	$ 341,029