Note 5 - Loans and Allowance for Loan Losses - Acquired Loans (Details) (Parentheticals) $ in Thousands	Dec. 31, 2018 USD ($)
Carrying amount, fair value adjustment	$ 70

Fair value adjustment of loans acquired in a business combination.

No definition available.

Name:	fusb_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesFairValueAdjustment
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant