Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member] - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 278,769	$ 254,742
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	255,694	232,592
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	255,694
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 255,964	$ 232,592
Total Capital (to Risk-Weighted Assets), Ratio	12.03%	14.38%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.04%	13.13%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.04%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.48%	6.52%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 185,320	$ 141,747
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	138,990	70,873
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	104,242
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 157,734	$ 142,630
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%	4.00%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	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	$ 231,650	$ 177,183
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	185,320	106,310
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	150,572
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 197,167	$ 178,287
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%	6.00%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	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

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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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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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