Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member]	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 448,695	$ 401,850
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	413,209	372,265
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	413,209	372,265
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 413,209	$ 372,265
Total Capital (to Risk-Weighted Assets), Ratio	0.1303	0.1357
Tier 1 Capital (to Risk-Weighted Assets), Ratio	0.1200	0.1257
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.00%	12.57%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	0.0641	0.0701
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 275,403	$ 236,830
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	206,553	177,622
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	154,914	133,217
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 257,763	$ 212,549
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0800	0.0800
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0600	0.0600
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.50%	4.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	0.0400	0.0400
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 344,254	$ 296,037
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	275,403	236,830
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	223,765	192,424
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 322,204	$ 265,686
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.1000	0.1000
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.0800	0.0800
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.50%	6.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	0.0500	0.0500

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