Basis of Financial Statement Presentation - Additional Information (Detail)	6 Months Ended
Jun. 30, 2017 Segment
Basis Of Presentation [Line Items]
Number of reportable segments	1
Century Bancorp Capital Trust II [Member]
Basis Of Presentation [Line Items]
Equity ownership interest	100.00%

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration