Parent Company Financial Statements - Statements of Cash Flows of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 9,925	$ 9,581	$ 8,998	$ 7,709	$ 39,699	$ 36,213	$ 22,301
Adjustments to reconcile net income to net cash provided by operating activities
Decrease (increase) in other assets									8,532	2,326	(16,310)
Net cash provided by operating activities									47,849	40,788	21,586
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(2,207)	(2,203)	(2,200)
Net cash provided by financing activities									271,322	337,644	293,037
Century Bancorp, Inc. [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									39,699	36,213	22,301
Adjustments to reconcile net income to net cash provided by operating activities
Undistributed income of subsidiary									(36,063)	(32,762)	(20,656)
Decrease (increase) in other assets									665	(158)	(6,498)
(Decrease) increase in liabilities									(180)	(1,808)	6,266
Net cash provided by operating activities									4,121	1,485	1,413
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(2,207)	(2,203)	(2,200)
Net cash provided by financing activities									(2,207)	(2,203)	(2,200)
Net increase (decrease) in cash									1,914	(718)	(787)
Cash and cash equivalents at beginning of year				$ 1,263				$ 1,981	1,263	1,981	2,768
Cash and cash equivalents at end of year	$ 3,177				$ 1,263				$ 3,177	$ 1,263	$ 1,981