Regulatory Matters - Additional Information (Details)	Dec. 31, 2019
Regulatory Matters
Capital Conservation Buffer, Initial Percentage	0.00%
Capital Conservation Buffer, Target percentage	2.50%
Capital Conservation Buffer, Percentage	2.50%

Represents the initial capital conservation buffer initiated in 2015 to be maintained for the purposes under the regulatory framework for prompt corrective action.

No definition available.

Name:	cnnb_CapitalConservationBufferInitialPercentage
Namespace Prefix:	cnnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant