XML 58 R52.htm IDEA: XBRL DOCUMENT
v3.25.3

Loans And Leases - Carrying Amount of Loans And Leases by Credit Quality Indicator and Year of Origination (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 | Dec. 31, 2024
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Total | $ 867,520 | $ 823,039
Commercial [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 53,260 | 47,502
2024 | 52,178 | 49,085
2023 | 38,087 | 125,939
2022 | 108,901 | 72,076
2021 | 25,349 | 63,529
Prior | 131,510 | 80,452
Total | 409,285 | 438,583
Current period gross charge-offs, 2023 | 2,215 | 54
Current period gross charge-offs, 2021 |  | 43
Current period gross charge-offs, Prior |  | 272
Current period gross charge-offs | 2,215 | 369
Commercial [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 52,733 | 47,502
2024 | 47,762 | 43,312
2023 | 35,243 | 125,593
2022 | 98,058 | 71,470
2021 | 25,349 | 63,048
Prior | 130,714 | 79,766
Total | 389,859 | 430,691
Commercial [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 70 |
2024 | 4,416 |
2023 | 540 | 346
2022 | 10,843 | 121
2021 | 0 | 334
Prior | 326 |
Total | 16,195 | 801
Commercial [Member] | Substandard [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 457 |
2024 |  | 2,489
2023 | 2,304 |
2022 |  | 485
2021 | 0 | 147
Prior | 470 | 686
Total | 3,231 | 3,807
Commercial [Member] | Doubtful [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2024 |  | 3,284
2023 | 0 |
Total | 0 | 3,284
Commercial [Member] | Construction, Land Development and Other Land Loans [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 31 | 852
2024 | 12,515 | 7,115
2023 | 16,304 | 45,087
2022 | 7,753 | 11,992
2021 | 1,852 | 41
Prior | 105 | 450
Total | 38,560 | 65,537
Commercial [Member] | Construction, Land Development and Other Land Loans [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 6 | 852
2024 | 8,099 | 4,626
2023 | 14,000 | 45,087
2022 | 7,753 | 11,931
2021 | 1,852 | 41
Prior | 105 | 450
Total | 31,815 | 62,987
Commercial [Member] | Construction, Land Development and Other Land Loans [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 25 |
2024 | 4,416 |
2022 |  | 61
Total | 4,441 | 61
Commercial [Member] | Construction, Land Development and Other Land Loans [Member] | Substandard [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2024 |  | 2,489
2023 | 2,304 |
Total | 2,304 | 2,489
Commercial [Member] | Secured By Multi family Residential Properties [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 13,601 | 7,941
2024 |  | 8,218
2023 | 381 | 42,077
2022 | 73,747 | 17,557
2021 | 1,025 | 5,592
Prior | 23,808 | 19,672
Total | 112,763 | 101,057
Commercial [Member] | Secured By Multi family Residential Properties [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 13,601 | 7,941
2024 | 201 | 8,218
2023 | 381 | 42,077
2022 | 65,771 | 17,557
2021 | 1,025 | 5,592
Prior | 23,808 | 19,672
Total | 104,787 | 101,057
Commercial [Member] | Secured By Multi family Residential Properties [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2022 | 7,976 |
Total | 7,976 |
Commercial [Member] | Secured by Non-residential Commercial Real Estate [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 35,648 | 25,251
2024 | 25,271 | 24,906
2023 | 16,761 | 35,246
2022 | 24,808 | 37,276
2021 | 11,257 | 55,008
Prior | 97,655 | 50,064
Total | 211,400 | 227,751
Current period gross charge-offs, Prior |  | 248
Current period gross charge-offs |  | 248
Commercial [Member] | Secured by Non-residential Commercial Real Estate [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 35,191 | 25,251
2024 | 25,271 | 24,906
2023 | 16,221 | 34,930
2022 | 21,959 | 36,793
2021 | 11,257 | 54,527
Prior | 96,966 | 49,681
Total | 206,865 | 226,088
Commercial [Member] | Secured by Non-residential Commercial Real Estate [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2023 | 540 | 316
2022 | 2,849 |
2021 |  | 334
Prior | 326 |
Total | 3,715 | 650
Commercial [Member] | Secured by Non-residential Commercial Real Estate [Member] | Substandard [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 457 |
2022 |  | 483
2021 | 0 | 147
Prior | 363 | 383
Total | 820 | 1,013
Commercial [Member] | Commercial and Industrial Loans [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 |  | 13,458
2024 |  | 8,846
2023 |  | 3,529
2022 |  | 5,251
2021 |  | 2,888
Prior |  | 10,266
Total |  | 44,238
Current period gross charge-offs, 2023 | 2,215 | 54
Current period gross charge-offs, 2021 |  | 43
Current period gross charge-offs, Prior |  | 24
Current period gross charge-offs | 2,215 | 121
Commercial [Member] | Commercial and Industrial Loans [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 |  | 13,458
2024 |  | 5,562
2023 |  | 3,499
2022 |  | 5,189
2021 |  | 2,888
Prior |  | 9,963
Total |  | 40,559
Commercial [Member] | Commercial and Industrial Loans [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2023 |  | 30
2022 |  | 60
Total |  | 90
Commercial [Member] | Commercial and Industrial Loans [Member] | Substandard [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2022 |  | 2
Prior |  | 303
Total |  | 305
Commercial [Member] | Commercial and Industrial Loans [Member] | Doubtful [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2024 |  | 3,284
Total |  | 3,284
Commercial [Member] | Commercial and Industrial Loans and Leases ("C&I") [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 3,980 |
2024 | 14,191 |
2023 | 4,641 |
2022 | 2,593 |
2021 | 11,215 |
Prior | 9,942 |
Total | 46,562 |
Commercial [Member] | Commercial and Industrial Loans and Leases ("C&I") [Member] | Pass [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 3,935 |
2024 | 14,191 |
2023 | 4,641 |
2022 | 2,575 |
2021 | 11,215 |
Prior | 9,835 |
Total | 46,392 |
Commercial [Member] | Commercial and Industrial Loans and Leases ("C&I") [Member] | Special Mention [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 45 |
2022 | 18 |
2021 | 0 |
Total | 63 |
Commercial [Member] | Commercial and Industrial Loans and Leases ("C&I") [Member] | Substandard [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Prior | 107 |
Total | 107 |
Commercial [Member] | Commercial and Industrial Loans and Leases ("C&I") [Member] | Doubtful [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2023 | 0 |
Total | 0 |
Consumer [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 131,866 | 61,502
2024 | 54,350 | 77,063
2023 | 64,066 | 93,004
2022 | 80,462 | 71,137
2021 | 61,104 | 48,075
Prior | 66,387 | 33,675
Total | 458,235 | 384,456
Current period gross charge-offs, 2025 | 10 | 21
Current period gross charge-offs, 2024 | 99 | 95
Current period gross charge-offs, 2023 | 203 | 279
Current period gross charge-offs, 2022 | 449 | 573
Current period gross charge-offs, 2021 | 278 | 332
Current period gross charge-offs, Prior | 394 | 145
Current period gross charge-offs | 1,433 | 1,445
Consumer [Member] | Performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 131,866 | 61,502
2024 | 54,327 | 77,063
2023 | 64,066 | 93,004
2022 | 80,462 | 71,137
2021 | 61,090 | 48,075
Prior | 65,721 | 32,062
Total | 457,532 | 382,843
Consumer [Member] | Non-performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2024 | 23 |
2021 | 14 |
Prior | 666 | 1,613
Total | 703 | 1,613
Consumer [Member] | Secured by 1-4 Family Residential Properties [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 4,206 | 4,559
2024 | 5,045 | 3,525
2023 | 2,979 | 18,060
2022 | 17,345 | 14,746
2021 | 14,093 | 5,884
Prior | 23,952 | 23,225
Total | 67,620 | 69,999
Current period gross charge-offs, Prior |  | 2
Current period gross charge-offs |  | 2
Consumer [Member] | Secured by 1-4 Family Residential Properties [Member] | Performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 4,206 | 4,559
2024 | 5,045 | 3,525
2023 | 2,979 | 18,060
2022 | 17,345 | 14,746
2021 | 14,093 | 5,884
Prior | 23,359 | 21,612
Total | 67,027 | 68,386
Consumer [Member] | Secured by 1-4 Family Residential Properties [Member] | Non-performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Prior | 593 | 1,613
Total | 593 | 1,613
Consumer [Member] | Direct Consumer [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 2,533 | 2,397
2024 | 1,354 | 1,077
2023 | 557 | 430
2022 | 185 | 487
2021 | 227 | 283
Prior | 143 | 100
Total | 4,999 | 4,774
Current period gross charge-offs, 2024 | 4 |
Current period gross charge-offs, 2023 |  | 1
Current period gross charge-offs, 2022 |  | 42
Current period gross charge-offs, 2021 | 1 | 3
Current period gross charge-offs, Prior |  | 16
Current period gross charge-offs | 5 | 62
Consumer [Member] | Direct Consumer [Member] | Performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 2,533 | 2,397
2024 | 1,354 | 1,077
2023 | 557 | 430
2022 | 185 | 487
2021 | 223 | 283
Prior | 143 | 100
Total | 4,995 | 4,774
Consumer [Member] | Direct Consumer [Member] | Non-performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2021 | 4 |
Prior | 0 |
Total | 4 |
Consumer [Member] | Indirect [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 125,127 |
2024 | 47,951 |
2023 | 60,530 |
2022 | 62,932 |
2021 | 46,784 |
Prior | 42,292 |
Total | 385,616 | 309,683
Current period gross charge-offs, 2025 | 10 |
Current period gross charge-offs, 2024 | 95 |
Current period gross charge-offs, 2023 | 203 |
Current period gross charge-offs, 2022 | 449 |
Current period gross charge-offs, 2021 | 277 |
Current period gross charge-offs, Prior | 394 |
Current period gross charge-offs | 1,428 |
Consumer [Member] | Indirect [Member] | Performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 | 125,127 |
2024 | 47,928 |
2023 | 60,530 |
2022 | 62,932 |
2021 | 46,774 |
Prior | 42,219 |
Total | 385,510 |
Consumer [Member] | Indirect [Member] | Non-performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2024 | 23 |
2021 | 10 |
Prior | 73 |
Total | $ 106 |
Consumer [Member] | Indirect Consumer [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 |  | 54,546
2024 |  | 72,461
2023 |  | 74,514
2022 |  | 55,904
2021 |  | 41,908
Prior |  | 10,350
Total |  | 309,683
Current period gross charge-offs, 2025 |  | 21
Current period gross charge-offs, 2024 |  | 95
Current period gross charge-offs, 2023 |  | 278
Current period gross charge-offs, 2022 |  | 531
Current period gross charge-offs, 2021 |  | 329
Current period gross charge-offs, Prior |  | 127
Current period gross charge-offs |  | 1,381
Consumer [Member] | Indirect Consumer [Member] | Performing [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
2025 |  | 54,546
2024 |  | 72,461
2023 |  | 74,514
2022 |  | 55,904
2021 |  | 41,908
Prior |  | 10,350
Total |  | $ 309,683

X
- Definition Allowance for loan and lease losses write offs current fiscal year. + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsCurrentFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Allowance for loan and lease losses write offs fiscal year before latest fiscal year. + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsFiscalYearBeforeLatestFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Allowance for loan and lease losses write offs five or more years before latest fiscal year. + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Allowance for loan and lease losses write offs four years before latest fiscal year. + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsFourYearsBeforeLatestFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Allowance for loan and lease losses write offs three years before latest fiscal year. + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsThreeYearsBeforeLatestFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Allowance for loan and lease losses write offs two years before latest fiscal year + References No definition available. + Details
Name: | fusb_AllowanceForLoanAndLeaseLossesWriteOffsTwoYearsBeforeLatestFiscalYear
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11B -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
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/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(4) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 8: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef -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 Reference 11: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 12: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 13: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 14: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (e)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 15: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (e)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 16: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(4) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 17: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 18: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 19: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 20: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 21: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 22: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 29 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-29 Reference 23: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -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 + Details
Name: | us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_SecuredBy1To4FamilyResidentialPropertiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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