Loans Receivable and Allowance for Credit Losses - Nonaccrual Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Jun. 30, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	$ 4,093	$ 24,245
Nonaccrual with an allowance for credit loss	18,178	0
Loans past due over 89 days still accruing	0	0
Construction, land & land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	0	0
Nonaccrual with an allowance for credit loss	0	0
Loans past due over 89 days still accruing	0	0
Other commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	4,029	6,827
Nonaccrual with an allowance for credit loss	0	0
Loans past due over 89 days still accruing	0	0
Owner-occupied commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	0	17,418
Nonaccrual with an allowance for credit loss	18,178	0
Loans past due over 89 days still accruing	0	0
Commercial industrial & agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	0	0
Nonaccrual with an allowance for credit loss	0	0
Loans past due over 89 days still accruing	0	0
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	64	0
Nonaccrual with an allowance for credit loss	0	0
Loans past due over 89 days still accruing	0	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual with no allowance for credit loss	0	0
Nonaccrual with an allowance for credit loss	0	0
Loans past due over 89 days still accruing	$ 0	$ 0

Financing Receivable, Excluding Accrued Interest, Nonaccrual, With Allowance

No definition available.

Name:	fcbm_FinancingReceivableExcludingAccruedInterestNonaccrualWithAllowance
Namespace Prefix:	fcbm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(c)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 4: 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

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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

- Definition

Amortized cost of financing receivable 90 days or more past due and still accruing. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-7

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-80

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fcbm_ConstructionLandAndLandDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fcbm_OtherCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fcbm_OwnerOccupiedCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fcbm_CommercialIndustrialAndAgriculturalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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