Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 329,666	$ 293,143
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	303,411	268,737
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	303,411	268,737
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 303,411	$ 268,737
Total Capital (to Risk-Weighted Assets), Ratio	12.70%	12.27%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.69%	11.25%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.69%	11.25%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.55%	6.02%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 207,707	$ 191,081
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	155,780	143,311
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	116,835	107,483
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 185,199	$ 178,469
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	$ 259,633	$ 238,851
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	207,707	191,081
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	168,762	155,253
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 231,499	$ 223,086
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%

Common Equity Tier One Risk Based Capital

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Common Equity Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets

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Common equity tier one risk based capital required to be well capitalized.

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Common Equity Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets

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Common Equity Tier One Risk Based Capital To Risk Weighted Assets

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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