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Investments - ISLP Forward Foreign Currency Exchange Contracts (Details)
€ in Thousands, £ in Thousands, kr in Thousands, kr in Thousands, $ in Thousands, $ in Thousands, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2022
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2022
USD ($)
Sep. 30, 2021
USD ($)
Dec. 31, 2021
USD ($)
Sep. 30, 2022
EUR (€)
Sep. 30, 2022
GBP (£)
Sep. 30, 2022
NOK (kr)
Sep. 30, 2022
AUD ($)
Sep. 30, 2022
CAD ($)
Sep. 30, 2022
DKK (kr)
Dec. 31, 2021
EUR (€)
Dec. 31, 2021
GBP (£)
Dec. 31, 2021
NOK (kr)
Dec. 31, 2021
AUD ($)
Dec. 31, 2021
CAD ($)
Dec. 31, 2021
DKK (kr)
Forward Foreign Currency Exchange Contracts                                  
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts $ (2,210) $ 6,080 $ 7,565 $ 26,685 $ 5,321                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 1,458, Purchased And POUND STERLING 1,100, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 1,458                        
Currency Sold | £                         £ 1,100        
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Feb. 18, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (31)                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 1,777, Purchased And POUND STERLING 1,530, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 1,777   $ 1,777                            
Currency Sold | £             £ 1,530                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Mar. 16, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 1,804, Purchased And CANADIAN DOLLARS 2,360, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 1,804   $ 1,804                            
Currency Sold                   $ 2,360              
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Mar. 03, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 1,990, Purchased And EURO 1,900, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 1,990   $ 1,990                            
Currency Sold | €           € 1,900                      
Counterparty Citibank   Citibank     Citibank Citibank Citibank Citibank Citibank Citibank            
Settlement Date     Jan. 09, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 10,773, Purchased And EURO 9,890, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 10,773   $ 10,773                            
Currency Sold | €           € 9,890                      
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     May 17, 2024                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 10,917, Purchased And POUND STERLING 9,440, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 10,917   $ 10,917                            
Currency Sold | £             £ 9,440                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Mar. 03, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 11,215, Purchased And POUND STERLING 9,000, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 11,215   $ 11,215                            
Currency Sold | £             £ 9,000                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Jun. 24, 2024                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 11,848, Purchased And POUND STERLING 9,890, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 11,848   $ 11,848                            
Currency Sold | £             £ 9,890                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Feb. 17, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 12,327, Purchased And AUSTRALIAN DOLLARS 17,960, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 12,327   $ 12,327                            
Currency Sold                 $ 17,960                
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Jul. 28, 2025                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 12,327, Purchased And EURO 10,510, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 12,327                        
Currency Sold | €                       € 10,510          
Counterparty         Citibank             Citibank Citibank Citibank Citibank Citibank Citibank
Settlement Date         Sep. 02, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 305                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 14,330, Purchased And EURO 12,550, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 14,330                        
Currency Sold | €                       € 12,550          
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Sep. 06, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 25                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 24,349, Purchased And POUND STERLING 19,320, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 24,349   $ 24,349                            
Currency Sold | £             £ 19,320                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Nov. 17, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 27,411, Purchased And POUND STERLING 20,700, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 27,411                        
Currency Sold | £                         £ 20,700        
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Sep. 06, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 563                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 29,087, Purchased And POUND STERLING 20,990, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 29,087                        
Currency Sold | £                         £ 20,990        
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Sep. 02, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 721                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 35,821, Purchased And POUND STERLING 25,700, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 35,821                        
Currency Sold | £                         £ 25,700        
Counterparty         Citibank             Citibank Citibank Citibank Citibank Citibank Citibank
Settlement Date         Feb. 18, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 1,035                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 37,234, Purchased And POUND STERLING 31,000, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 37,234   $ 37,234                            
Currency Sold | £             £ 31,000                    
Counterparty Citibank   Citibank     Citibank Citibank Citibank Citibank Citibank Citibank            
Settlement Date     Jan. 09, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 4,754, Purchased And EURO 3,251, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 4,754                        
Currency Sold | €                       € 3,251          
Counterparty         Citibank             Citibank Citibank Citibank Citibank Citibank Citibank
Settlement Date         Sep. 06, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (1,036)                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 4,945, Purchased And NORWEGIAN KRONE 48,400, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 4,945   $ 4,945                            
Currency Sold | kr               kr 48,400                  
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Feb. 17, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 41,180, Purchased And EURO 40,810, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 41,180   $ 41,180                            
Currency Sold | €           € 40,810                      
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Mar. 03, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 481, Purchased And AUSTRALIAN DOLLARS 410, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 481                        
Currency Sold                             $ 410    
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Mar. 02, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 183                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 5,526, Purchased And AUSTRALIAN DOLLARS 7,750, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 5,526   $ 5,526                            
Currency Sold                 $ 7,750                
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Aug. 04, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 6,138, Purchased And POUND STERLING 5,000, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 6,138   $ 6,138                            
Currency Sold | £             £ 5,000                    
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Aug. 04, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 6,581, Purchased And EURO 6,400, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 6,581   $ 6,581                            
Currency Sold | €           € 6,400                      
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Jan. 09, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 6,954, Purchased And POUND STERLING 5,260, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 6,954                        
Currency Sold | £                         £ 5,260        
Counterparty         Citibank             Citibank Citibank Citibank Citibank Citibank Citibank
Settlement Date         Feb. 23, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 166                        
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 7,894, Purchased And AUSTRALIAN DOLLARS 11,440, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 7,894   $ 7,894                            
Currency Sold                 $ 11,440                
Counterparty bcsf:BankOfNewYorkMellonMember   bcsf:BankOfNewYorkMellonMember     bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember            
Settlement Date     Mar. 03, 2023                            
Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 75,862, Purchased And EURO 63,360, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 75,862                        
Currency Sold | €                       € 63,360          
Counterparty         bcsf:BankOfNewYorkMellonMember             bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember bcsf:BankOfNewYorkMellonMember
Settlement Date         Sep. 02, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 3,390                        
International Senior Loan Program, LLC                                  
Forward Foreign Currency Exchange Contracts                                  
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 3,968   $ 1,800                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: AUSTRALIAN DOLLARS 189, Purchased, EURO 121, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased                             $ 189    
Currency Sold | €                       € 121          
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: AUSTRALIAN DOLLARS 731, Purchased, US DOLLARS 532, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased                             $ 731    
Currency Sold         $ 532                        
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 1,099, Purchased, BRITISH POUNDS 940, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 1,099                      
Currency Sold | £             £ 940                    
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Feb. 17, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 27                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 1,932, Purchased, AUSTRALIAN DOLLARS 2,872, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 1,932                      
Currency Sold                 $ 2,872                
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 56                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 15,594, Purchased, US DOLLARS 18,205, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 15,594          
Currency Sold         $ 18,205                        
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (477)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 2,038, Purchased, AUSTRALIAN DOLLARS 3,166, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 2,038          
Currency Sold                             $ 3,166    
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 16                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 2,530, Purchased, US DOLLARS 2,610, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 2,530                      
Currency Sold $ 2,610   $ 2,610                            
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Jan. 09, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ (130)                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 23,474, Purchased, US DOLLARS 24,060, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 23,474                      
Currency Sold $ 24,060   $ 24,060                            
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ (1,050)                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 479, Purchased, CANADIAN DOLLARS 640, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 479                      
Currency Sold                   $ 640              
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Mar. 27, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 11                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 5,224, Purchased, BRITISH POUNDS 4,459, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 5,224                      
Currency Sold | £             £ 4,459                    
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 140                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 5,379, Purchased, US DOLLARS 6,110, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 5,379          
Currency Sold         $ 6,110                        
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 5                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 669, Purchased, CANADIAN DOLLARS 885, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 669                      
Currency Sold                   $ 885              
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 16                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 683, Purchased, CANADIAN DOLLARS 982, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 683          
Currency Sold                               $ 982  
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (2)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 7,922, Purchased, US DOLLARS 7,880, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 7,922                      
Currency Sold $ 7,880   $ 7,880                            
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Dec. 09, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ (80)                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 8,236, Purchased, BRITISH POUNDS 6959, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 8,236          
Currency Sold | £                         £ 6,959        
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (57)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 829, Purchased, NORWEGIAN KRONE 8,444, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 829                      
Currency Sold | kr               kr 8,444                  
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 36                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 874, Purchased, DANISH KRONE 6,502, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €           € 874                      
Currency Sold | kr                     kr 6,502            
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 884, Purchased, NORWEGIAN KRONE 8,626, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 884          
Currency Sold | kr                           kr 8,626      
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 27                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: EURO 892_20211231, Purchased, DANISH KRONE 6,643, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased | €                       € 892          
Currency Sold | kr                                 kr 6,643
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (1)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 1,801, Purchased, CANADIAN DOLLARS 2,456, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 1,801   $ 1,801                            
Currency Sold                   $ 2,456              
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Mar. 27, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 19                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 2,650, Purchased, CANADIAN DOLLARS 3,425, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 2,650   $ 2,650                            
Currency Sold                   $ 3,425              
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 173                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 20,718, Purchased, BRITISH POUNDS 17,261, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 20,718   $ 20,718                            
Currency Sold | £             £ 17,261                    
Counterparty bcsf:GoldmanSachsMember   bcsf:GoldmanSachsMember     bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 1,438                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 3,087, Purchased, CANADIAN DOLLARS 3,803, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 3,087                        
Currency Sold                               $ 3,803  
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 75                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 3,285, Purchased, NORWEGIAN KRONE 32,686, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 3,285   $ 3,285                            
Currency Sold | kr               kr 32,686                  
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 282                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 3,466, Purchased, DANISH KRONE 25,168, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 3,466   $ 3,466                            
Currency Sold | kr                     kr 25,168            
Counterparty bcsf:StandardCharteredBankMember   bcsf:StandardCharteredBankMember     bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 148                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 3,993, Purchased, NORWEGIAN KRONE 33,392, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 3,993                        
Currency Sold | kr                           kr 33,392      
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 209                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 33,462, Purchased, EURO 28,674, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 33,462                        
Currency Sold | €                       € 28,674          
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 863                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 37,224, Purchased, BRITISH POUNDS 26,939, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 37,224                        
Currency Sold | £                         £ 26,939        
Counterparty         bcsf:GoldmanSachsMember             bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 756                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 4,033, Purchased, DANISH KRONE 25,714, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 4,033                        
Currency Sold | kr                                 kr 25,714
Counterparty         bcsf:StandardCharteredBankMember             bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember bcsf:StandardCharteredBankMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 102                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 45,228, Purchased, EURO 44,150, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 45,228   $ 45,228                            
Currency Sold | €           € 44,150                      
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 1,952                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 5,022, Purchased, EURO 4,420, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 5,022                        
Currency Sold | €                       € 4,420          
Counterparty         bcsf:GoldmanSachsMember             bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember bcsf:GoldmanSachsMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (3)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 5,084, Purchased, EURO 5,150, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 5,084   $ 5,084                            
Currency Sold | €           € 5,150                      
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Jan. 09, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ (3)                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 5,938, Purchased, BRITISH POUNDS 4,970, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 5,938   $ 5,938                            
Currency Sold | £             £ 4,970                    
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Feb. 17, 2023                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 386                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 609, Purchased, EURO 540, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 609                        
Currency Sold | €                       € 540          
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (5)                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 7,663, Purchased, AUSTRALIAN DOLLARS 11,118, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased $ 7,663   $ 7,663                            
Currency Sold                 $ 11,118                
Counterparty bcsf:MorganStanleyMember   bcsf:MorganStanleyMember     bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember            
Settlement Date     Oct. 18, 2022                            
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts     $ 547                            
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 9,207, Purchased, AUSTRALIAN DOLLARS 12,254, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 9,207                        
Currency Sold                             $ 12,254    
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ 299                        
International Senior Loan Program, LLC | Open Forward Foreign Currency Contract, Identifier [Axis]: US DOLLARS 948, Purchased, EURO 840, Sold                                  
Forward Foreign Currency Exchange Contracts                                  
Currency Purchased         $ 948                        
Currency Sold | €                       € 840          
Counterparty         bcsf:MorganStanleyMember             bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember bcsf:MorganStanleyMember
Settlement Date         Jan. 21, 2022                        
Net change in unrealized appreciation (depreciation) on forward currency exchange contracts         $ (7)