Fair value portion of federal funds sold.

No definition available.

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock.

+ References

No definition available.

+ Details

Name:	us-gaap_InvestmentInFederalHomeLoanBankStockFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Note 16 - Fair Value of Financial Instruments - Estimated Fair Value and Related Carrying or Notional Amounts of Financial Instruments (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Assets:
Investment securities available-for-sale	$ 127,185,000	$ 132,487,000
Investment securities held-to-maturity	20,467,000	20,852,000	$ 20,467,000
Reported Value Measurement [Member]
Assets:
Cash and cash equivalents	60,046,000	49,599,000
Investment securities available-for-sale	127,185,000	132,487,000
Investment securities held-to-maturity	20,840,000	21,462,000
Federal funds sold	15,080,000	8,354,000
Federal Home Loan Bank stock	713,000	703,000
Loans, net of allowance for loan and lease losses	502,760,000	514,867
Liabilities:
Deposits	703,361,000	704,725,000
Short-term borrowings		527,000
Estimate of Fair Value Measurement [Member]
Assets:
Cash and cash equivalents	60,046,000	49,599,000
Investment securities available-for-sale	127,185,000	132,487,000
Investment securities held-to-maturity	20,467,000	20,852,000
Federal funds sold	15,080,000	8,354,000
Federal Home Loan Bank stock	713,000	703,000
Loans, net of allowance for loan and lease losses	512,378,000	516,420
Liabilities:
Deposits	702,185,000	702,832,000
Short-term borrowings		527,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Cash and cash equivalents	60,046,000	49,599,000
Investment securities available-for-sale
Investment securities held-to-maturity
Federal funds sold
Federal Home Loan Bank stock
Loans, net of allowance for loan and lease losses
Liabilities:
Deposits
Short-term borrowings
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Assets:
Cash and cash equivalents
Investment securities available-for-sale	127,185,000	132,487,000
Investment securities held-to-maturity	20,467,000	20,852,000
Federal funds sold	15,080,000	8,354,000
Federal Home Loan Bank stock
Loans, net of allowance for loan and lease losses
Liabilities:
Deposits	702,185,000	702,832,000
Short-term borrowings		527,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member]
Assets:
Cash and cash equivalents
Investment securities available-for-sale
Investment securities held-to-maturity
Federal funds sold
Federal Home Loan Bank stock	713,000	703,000
Loans, net of allowance for loan and lease losses	512,378,000	516,420
Liabilities:
Deposits
Short-term borrowings