Note 2 - Investments (Details Textual)	Jun. 30, 2017
Percentage of Capital Surplus Equal to Federal Reserve Bank Stock Required to Hold	6.00%

The percentage of capital surplus, which is defined as additional paid-in capital less any gains (losses) on available-for-sale securities as of the current period, equal to Federal Reserve Bank stock that the company is required to hold.

No definition available.

Name:	cfnb_PercentageOfCapitalSurplusEqualToFederalReserveBankStockRequiredToHold
Namespace Prefix:	cfnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant