Name:	bcsf_DebtInstrumentBasisSpreadOnBaseRateOption
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Data Type:	dtr-types:percentItemType
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Period Type:	duration

The threshold borrowing base of debt instrument under base rate option.

No definition available.

Name:	bcsf_DebtInstrumentThresholdAlternateBaseRate
Namespace Prefix:	bcsf_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

The threshold borrowing base of debt instrument.

No definition available.

Name:	bcsf_DebtInstrumentThresholdBorrowingBase
Namespace Prefix:	bcsf_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

The amount of borrowing available under credit facility after accordion provisions.

No definition available.

Name:	bcsf_LineOfCreditMaximumCapacityAfterAccordionProvisions
Namespace Prefix:	bcsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
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Data Type:	dtr-types:percentItemType
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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.

No definition available.

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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(16))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Name:	us-gaap_LineOfCredit
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

No definition available.

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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

Name:	us-gaap_DebtInstrumentAxis=bcsf_SumitomoCreditFacilityMember
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Name:	srt_StatementScenarioAxis=bcsf_BorrowingBaseEqualToOrGreaterThanProductOf1.60AndRevolvingCreditExposureMember
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Name:	us-gaap_VariableRateAxis=us-gaap_FederalFundsEffectiveSwapRateMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_VariableRateAxis=bcsf_OneMonthEurocurrencyRateMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_VariableRateAxis=bcsf_AlternateBaseRateMember
Namespace Prefix:
Data Type:	na
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Name:	srt_StatementScenarioAxis=bcsf_BorrowingBaseLessThanProductOf1.60AndRevolvingCreditExposureMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
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