Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member] - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 401,850	$ 364,744
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	372,265	336,201
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	372,265	336,201
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 372,265	$ 336,201
Total Capital (to Risk-Weighted Assets), Ratio	13.57%	13.24%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.57%	12.21%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.57%	12.21%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	7.01%	6.68%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 236,830	$ 220,335
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	177,622	165,251
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	133,217	123,938
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 212,549	$ 201,228
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	6.00%	6.00%
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	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 296,037	$ 275,419
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	236,830	220,335
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	192,424	179,022
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 265,686	$ 251,535
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	8.00%	8.00%
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	5.00%	5.00%

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The amount of total capital as defined in the regulations.

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