Parent Company Financial Statements - Statements of Income 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
Income:
Other income									$ 4,456	$ 3,764	$ 3,906
Total interest income	$ 40,518	$ 39,852	$ 39,692	$ 39,077	$ 37,453	$ 34,765	$ 33,408	$ 31,430	159,139	137,056	113,436
Interest expense	15,187	16,082	16,442	15,639	13,748	11,561	10,209	8,962	63,350	44,480	27,820
Operating expenses	18,212	17,462	18,264	18,190	17,185	17,348	17,159	18,001	72,129	69,693	67,119
Income before income taxes	11,258	10,519	9,733	9,300	10,234	10,025	9,312	8,210	40,809	37,781	33,259
Benefit from income taxes	526	435	267	(118)	309	444	314	501	1,110	1,568	10,958
Net income	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 9,925	$ 9,581	$ 8,998	$ 7,709	39,699	36,213	22,301
Century Bancorp, Inc. [Member]
Income:
Dividends from subsidiary									5,000	4,750	2,500
Interest income from deposits in bank											1
Other income									65	53	34
Total interest income									5,065	4,803	2,535
Interest expense									1,577	1,474	1,121
Operating expenses									215	225	209
Income before income taxes									3,273	3,104	1,205
Benefit from income taxes									(363)	(347)	(440)
Income before equity in undistributed income of subsidiary									3,636	3,451	1,645
Equity in undistributed income of subsidiary									36,063	32,762	20,656
Net income									$ 39,699	$ 36,213	$ 22,301