Additional committed impaired loans.

No definition available.

Name:	bcow_AdditionalCommittedImpairedLoans
Namespace Prefix:	bcow_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable, deferral amount.

+ References

No definition available.

+ Details

Name:	bcow_FinancingReceivableDeferralAmount
Namespace Prefix:	bcow_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Financing Receivable, Interest Rate

+ References

No definition available.

+ Details

Name:	bcow_FinancingReceivableInterestRate
Namespace Prefix:	bcow_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Number Of Contracts

+ References

No definition available.

+ Details

Name:	bcow_FinancingReceivableNumberOfContracts
Namespace Prefix:	bcow_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Participation loans transferred.

+ References

No definition available.

+ Details

Name:	bcow_ParticipationLoansTransferred
Namespace Prefix:	bcow_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage Of Loan Guarantee By Small Business Administration

+ References

No definition available.

+ Details

Name:	bcow_PercentageOfLoanGuaranteeBySmallBusinessAdministration
Namespace Prefix:	bcow_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of deposits held by the entity that have been reclassified as loan balances receivable, for example, but not limited to, overdrafts receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

+ Details

Name:	us-gaap_DepositLiabilitiesReclassifiedAsLoansReceivable1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

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

- Definition

Threshold period for when financing receivable is considered past due, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Excludes threshold period past due to write off as uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447

+ Details

Name:	us-gaap_FinancingReceivableThresholdPeriodPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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

No definition available.

+ Details

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

- 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

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bcow_PppLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	bcow_UnexpectedEventAxis=bcow_CovidNineteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=bcow_OtherFinancialInstitutionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans - Additional Information (Detail) - USD ($)	12 Months Ended
Loans - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Aug. 08, 2020
Loans and Leases Receivable Disclosure [Line Items]
Loans 90 days or more past due and accruing interest	$ 0	$ 0
Internal risk ratings of loans	331,598,000	312,370,000
Additional committed impared loans	0	0
Loans modified as troubled debt restructurings	0	0
Overdrawn deposit account reclassified as loan	141,000	114,000
Loans and leases receivable, net amount	329,073,000	$ 310,674,000
PPP loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Interest Rate			1.00%
PPP loans [Member] \| Commercial Loan [Member]
Loans and Leases Receivable Disclosure [Line Items]
Loans and leases receivable, net amount	$ 17,200,000
Percentage Of Loan Guarantee By Small Business Administration	100.00%
Financing Receivable, Number Of Contracts	246
Covid Nineteen [Member] \| Payment Deferral [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing recievable threshold past due		30 days
Loans and leases receivable, net amount	$ 14,100,000
Financing receivable deferral amount	308
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Internal risk ratings of loans	0	$ 0
Troubled debt restructurings within past twelve months for which there was a default	0	0
Other Financial Institutions [Member]
Loans and Leases Receivable Disclosure [Line Items]
Participation loans transferred	$ 29,600,000	$ 26,200,000