Available for Sale Securities - Summary of Amortized Costs and Fair Value of Securities Available-for-sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	$ 71,229	$ 67,900
Securities Available for Sale, Gross Unrealized Gains	542	32
Securities Available for Sale, Gross Unrealized Losses	(396)	(2,201)
Securities Available for Sale, Fair Value	71,375	65,731
Obligations of States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	9,779	11,348
Securities Available for Sale, Gross Unrealized Gains	67	25
Securities Available for Sale, Gross Unrealized Losses	(20)	(204)
Securities Available for Sale, Fair Value	9,826	11,169
Government-sponsored Mortgage-backed Securities [Member]
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	56,975	52,363
Securities Available for Sale, Gross Unrealized Gains	416	4
Securities Available for Sale, Gross Unrealized Losses	(357)	(1,992)
Securities Available for Sale, Fair Value	57,034	50,375
Corporate Collateralized Mortgage Obligations [Member]
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	284	410
Securities Available for Sale, Gross Unrealized Gains	5	1
Securities Available for Sale, Gross Unrealized Losses		(1)
Securities Available for Sale, Fair Value	289	410
Asset-backed Securities [Member]
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	2,484	3,530
Securities Available for Sale, Gross Unrealized Gains		2
Securities Available for Sale, Gross Unrealized Losses	(19)	(1)
Securities Available for Sale, Fair Value	2,465	3,531
Certificates of Deposit [Member]
Marketable Securities [Line Items]
Securities Available for Sale, Amortized Cost	1,707	249
Securities Available for Sale, Gross Unrealized Gains	54
Securities Available for Sale, Gross Unrealized Losses		(3)
Securities Available for Sale, Fair Value	$ 1,761	$ 246

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
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

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=bcow_ObligationsOfStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=bcow_GovernmentSponsoredMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=bcow_CorporateCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: