Note 3 - Restricted Cash Balances (Details Textual) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Average Reserves Maintained with Federal Reserve Bank	$ 1.6	$ 1.6
Compensating Balances Maintained at Correspondent Banks	$ 1.2	$ 2.8

Represents the amount of compensating balances maintained at correspondent banks, most of which are not required, but are used to offset specific charges for services.

No definition available.

Name:	fncb_CompensatingBalancesMaintainedAtCorrespondentBanks
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant