Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) (Century Bancorp Capital Trust II [Member], USD $)	Dec. 31, 2014	Dec. 31, 2013
Century Bancorp Capital Trust II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 254,742	$ 230,038
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	232,592	209,360
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	232,592	209,360
Total Capital (to Risk-Weighted Assets), Ratio	14.38%	13.91%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	13.13%	12.66%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.52%	6.00%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	141,747	132,338
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	70,873	66,169
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	142,630	139,467
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.00%	4.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	177,183	165,422
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	106,310	99,253
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 178,287	$ 174,334
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.00%	6.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	5.00%	5.00%