XML 59 R50.htm IDEA: XBRL DOCUMENT
v3.23.2

Loans and Allowance for Credit Losses - Additional Information (Details) - USD ($) | 6 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2023 | Dec. 31, 2022 | Mar. 31, 2023 | Jan. 01, 2023 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Loans and Leases Receivable, Related Parties, Ending Balance | $ 1,400,000 | $ 200,000 |  |  |  |  |
Loans and Leases Receivable, Related Parties, Additions |  | 0 |  |  |  |  |
Loans and Leases Receivable, Related Parties, Proceeds | 0 | 100,000 |  |  |  |  |
Transition adjustment increase in allowance for credit losses |  |  |  | $ 2,100,000 |  |  |
Decrease to retained earnings | 85,725,000 | 85,135,000 | $ 84,757,000 |  | $ 82,576,000 | $ 87,807,000 | $ 90,064,000
Minimum commitment required for evaluating idividually | 500,000 |  |  |  |  |  |
Commitment required for evaluating individually at management discretion | 500,000 |  |  |  |  |  |
Loans and leases, allowance for credit losses | 11,536,000 | 9,422,000 |  |  | 8,751,000 |  | 8,320,000
Retained Earnings [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Decrease to retained earnings | 106,157,000 | 104,460,000 | $ 104,427,000 |  | $ 100,838,000 | $ 99,604,000 | $ 98,428,000
Impact of Adopting CECL [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Transition adjustment increase in allowance for credit losses |  |  |  | 2,400,000 |  |  |
Decrease to retained earnings | (1,811,000) |  |  |  |  |  |
Loans and leases, allowance for credit losses | 2,123,000 |  |  | 2,100,000 |  |  |
Additional increase to allowance for credit losses on loans and leases | 500,000 |  |  |  |  |  |
Impact of Adopting CECL [Member] | Retained Earnings [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Decrease to retained earnings | (1,811,000) |  |  | 1,800,000 |  |  |
Reserve for Unfunded Commitments [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Transition adjustment increase in allowance for credit losses |  |  |  | 300,000 |  |  |
Loans and leases, allowance for credit losses | $ 400,000 | $ 0 |  |  |  |  |
Reserve for Unfunded Commitments [Member] | Impact of Adopting CECL [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Loans and leases, allowance for credit losses |  |  |  | $ 300,000 |  |  |
Real Estate [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Percentage of Loan Portfolio | 53.40% | 53.00% |  |  |  |  |
Pledged as Collateral [Member] | FHLB Borrowings [Member] |  |  |  |  |  |  |
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items] |  |  |  |  |  |  |
Loans pledged as collateral | $ 106,800,000 | $ 100,200,000 |  |  |  |  |

X
- Definition Additional increase to allowance for loan and lease losses. + References No definition available. + Details
Name: | fusb_AdditionalIncreaseToAllowanceForLoanAndLeaseLosses
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Commitment required for evaluating individually at management discretion. + References No definition available. + Details
Name: | fusb_CommitmentRequiredForEvaluatingIndividuallyAtManagementDiscretion
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Minimum commitment required for evaluating idividually. + References No definition available. + Details
Name: | fusb_MinimumCommitmentRequiredForEvaluatingIdividually
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Percentage of loan portfolio. + References No definition available. + Details
Name: | fusb_PercentageOfLoanPortfolio
Namespace Prefix: | fusb_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Transition adjustment increase in loan loss reserves. + References No definition available. + Details
Name: | fusb_TransitionAdjustmentIncreaseInLoanLossReserves
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-13 + Details
Name: | us-gaap_DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481696/310-30-50-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 11B -Subparagraph (c)(1) -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 11B -Subparagraph (g) -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Name Accounting Standards Codification -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480418/310-10-S99-4 + Details
Name: | us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483326/850-10-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedParties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reflects the amount of new advances made to related parties in the form of loan and lease receivables. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483326/850-10-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of cash inflow from related parties which reduce the amount of loans and leases receivable due from related parties. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 12 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-12 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(7)(e)(1)(i)) -SubTopic 210 -Topic 942 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amortized cost, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(5)(b)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481990/310-10-45-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-5 + Details
Name: | us-gaap_NotesReceivableNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of equity (deficit) attributable to parent and noncontrolling interest. Excludes temporary equity. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 24 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483421/250-10-45-24 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 23 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483421/250-10-45-23 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483421/250-10-45-5 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-5 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(iii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(iv) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (i)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a)(3)(iii)(03) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (f)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (f)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (d)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (f)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (f)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-1 Reference 23: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (e)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-7 Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482765/220-10-50-5 Reference 26: http://www.xbrl.org/2003/role/disclosureRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481674/830-30-50-1 Reference 27: http://www.xbrl.org/2003/role/disclosureRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481694/830-30-45-17 Reference 28: http://www.xbrl.org/2003/role/disclosureRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 20 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481694/830-30-45-20 Reference 29: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480167/946-830-55-11 Reference 30: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 205 -Name Accounting Standards Codification -Section 45 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480767/946-205-45-3 Reference 31: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 505 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481004/946-505-50-3 Reference 32: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(19)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1 Reference 33: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.6-05(4)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479617/946-210-S99-2 Reference 34: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.6-09(4)(b)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483575/946-220-S99-3 Reference 35: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.6-09(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483575/946-220-S99-3 Reference 36: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.6-09(7)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483575/946-220-S99-3 Reference 37: http://www.xbrl.org/2003/role/disclosureRef -Topic 505 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1 Reference 38: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(g)(1)(ii)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 Reference 39: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 Reference 40: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 28 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482907/825-10-50-28 Reference 41: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481231/810-10-45-15 Reference 42: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 16 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481231/810-10-45-16 Reference 43: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4I -Publisher FASB -URI https://asc.fasb.org//1943274/2147481175/810-10-55-4I + Details
Name: | us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnfundedLoanCommitmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fusb_RealEstate1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PledgingPurposeAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |