Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member] - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 293,143	$ 278,769
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	268,737	255,694
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	268,737	255,694
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 268,737	$ 255,964
Total Capital (to Risk-Weighted Assets), Ratio	12.27%	12.03%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.25%	11.04%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.25%	11.04%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.02%	6.48%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 191,081	$ 185,320
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	143,311	138,990
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	107,483	104,242
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 178,469	$ 157,734
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	$ 238,851	$ 231,650
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	191,081	185,320
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	155,253	150,572
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 223,086	$ 197,167
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

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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 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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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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