The period for decrease in borrowing capacity in line of credit.

No definition available.

Name:	bcsf_LineOfCreditDecreaseInBorrowingCapacityPeriod
Namespace Prefix:	bcsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) in borrowing capacity of the credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	bcsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The extension period of maturity of line of credit.

+ References

No definition available.

+ Details

Name:	bcsf_LineOfCreditMaturityExtensionPeriod
Namespace Prefix:	bcsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The period for repayment of line of credit following the closing of public offering.

+ References

No definition available.

+ Details

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Namespace Prefix:	bcsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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_LineOfCreditFacilityLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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 210.5-02.19(b),22(b))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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

- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(f))
-URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

+ Details

Name:	us-gaap_RepaymentsOfLinesOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=bcsf_JpmCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=bcsf_PeriodFromJanuary292020ToJuly292020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Debt - JPM Credit Facility (Details) - USD ($) $ in Millions						3 Months Ended	12 Months Ended
	Jul. 02, 2020	Jun. 18, 2020	Mar. 20, 2020	Jan. 29, 2020	Apr. 30, 2019	Mar. 31, 2020	Dec. 31, 2020
Debt
Increase (decrease) in borrowing capacity			$ 50.0
Period for decrease in borrowing capacity			90 days
Prepayment of advances		$ 50.0
Minimum
Debt
Margin spread rate (as a percent)						2.50%
Unused commitment fee (as a percent)	0.375%
Maximum
Debt
Margin spread rate (as a percent)						3.25%
Unused commitment fee (as a percent)	0.75%
JPM credit facility
Debt
Maximum borrowing capacity	$ 450.0			$ 500.0	$ 666.6
Margin spread rate (as a percent)				2.375%	2.75%
Period for repayment			2 days
Extension period	1 year
Unused commitment fee (as a percent)							0.75%
JPM credit facility \| LIBOR
Debt
Margin spread rate (as a percent)					2.75%		2.375%
JPM credit facility \| Minimum
Debt
Maximum borrowing capacity					$ 466.6
JPM credit facility \| Minimum \| Period from January 29, 2020 to July 29, 2020
Debt
Maximum borrowing capacity				$ 300.0
JPM credit facility \| Minimum \| Period after July 29, 2020 till end of reinvestment period
Debt
Maximum borrowing capacity				$ 350.0