v3.23.3
Portfolio Investments - Narrative (Details)
6 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2023
USD ($)
member
borrower
Sep. 30, 2022
USD ($)
borrower
Sep. 30, 2021
USD ($)
Aug. 15, 2023
May 18, 2021
Sep. 30, 2020
USD ($)
Feb. 25, 2020
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investment owned, percent of net assets   190.82% 200.24%          
Investments at fair value   $ 2,892,420,000 $ 2,494,111,000          
Number of borrowers | borrower   143 149          
Money market funds, percent of net assets   9.60% 2.10%          
Cash, cash equivalents and restricted cash $ 31,635,000 $ 145,539,000 $ 26,364,000 $ 31,635,000     $ 39,096,000  
Restricted cash 2,301,000 9,089,000 2,836,000 2,301,000        
Net realized gains (losses)   (33,155,000) 17,179,000 26,420,000        
Net unrealized appreciation (depreciation)   (28,555,000) (136,248,000) 114,519,000        
Credit facilities payable   710,000,000 700,000,000          
Assets   3,217,839,000 2,646,313,000          
Unfunded commitments   232,740,000 224,239,000          
Investment owned, at cost   3,044,119,000 2,617,754,000          
SLF JV I, LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   141,500,000 117,000,000          
OCSI Glick JV LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 50,000,000 $ 50,300,000          
Investment Owned, At Fair Value                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investment owned, percent of net assets   190.82% 200.24%          
Investments at fair value   $ 2,892,420,000 $ 2,494,111,000          
Investment Owned, At Fair Value | Investment Type Concentration Risk                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Concentration risk   100.00% 100.00%          
Senior secured debt                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investment owned, percent of net assets   165.01% 173.93%          
Investments at fair value   $ 2,501,385,000 $ 2,166,409,000          
Investment owned, at cost   $ 2,594,640,000 $ 2,227,245,000          
Senior secured debt | Investment Owned, At Fair Value | Investment Type Concentration Risk                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Concentration risk   86.47% 86.86%          
Subordinated debt securities, including JV debt instruments                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 218,390,000 $ 203,453,000          
Subordinated debt securities, including JV debt instruments | Investment Owned, At Fair Value | Investment Type Concentration Risk                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Concentration risk   7.50% 8.10%          
SLF JV I                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 141,500,000 $ 117,000,000          
SLF JV I Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   112,700,000 96,300,000          
Investment owned, at cost   112,700,000 96,300,000          
Interest income   $ 12,700,000 8,000,000 7,400,000        
Investment, basis spread, variable rate   7.00%            
Investment, interest rate, floor   1.00%            
SLF JV I, LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 28,900,000 20,700,000          
Investment owned, at cost   54,800,000 49,300,000          
Dividend income   4,200,000 2,900,000 900,000        
OCSI Glick JV LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   50,000,000 50,300,000          
Investment owned, at cost   50,300,000 50,200,000          
Glick JV Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Interest income 2,400,000 $ 6,700,000 4,700,000          
Investment, basis spread, variable rate   4.50%            
2025 Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Interest rate, percentage   3.50%           3.50%
2027 Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Interest rate, percentage   2.70%       2.70%    
2029 Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Interest rate, percentage   7.10%     7.10%      
The JVs                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Money market funds, percent of net assets   9.60%            
Cash, cash equivalents and restricted cash   $ 145,500,000            
Restricted cash   9,100,000            
SLF JV I                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 322,179,000 $ 359,625,000          
Number of borrowers | borrower   48 60          
Restricted cash   $ 2,987,000 $ 5,642,000          
Number of board of directors | member   4            
Number of board of directors selected by company | member   2            
Number of board of directors selected by counterparty | member   2            
Number of investment committee members selected by company | member   1            
Number of investment committee members selected by counterparty | member   1            
Investment owned, ownership percentage   87.50% 87.50%          
Credit facilities payable   $ 149,000,000 $ 230,000,000          
Assets   376,104,000 385,227,000          
Financial support funded by company and counterparty   190,500,000 165,500,000          
Financial support funded by company for investee   166,700,000 144,800,000          
Unfunded commitments   13,100,000 35,000,000          
Investment owned, at cost   331,808,000 382,673,000          
Interest income   38,984,000 24,014,000 20,018,000        
Investments sold   31,800,000 9,700,000 48,000,000        
Proceeds from sale of investments   30,600,000 9,700,000 47,200,000        
Gain (loss) on sale of investments   (200,000) 500,000 $ 2,500,000        
SLF JV I | SLF JV I Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Unfunded commitments   9,800,000 26,200,000          
Financial support funded during the period   16,400,000            
SLF JV I | SLF JV I, LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Unfunded commitments   3,300,000 8,800,000          
Financial support funded during the period   5,500,000            
SLF JV I | SLF JV I Facility | Revolving Credit Facility | Line of Credit                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Borrowing capacity   270,000,000 $ 270,000,000          
Credit facilities payable   $ 149,000,000            
SLF JV I | SLF JV I Facility | Revolving Credit Facility | Line of Credit | Until November 3, 2023 | Secured Overnight Financing Rate                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Debt instrument, basis spread, variable rate   2.00%            
SLF JV I | Kemper                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investment owned, ownership percentage   12.50% 12.50%          
OCSI Glick JV LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 123,343,000 $ 133,144,000          
Number of borrowers | borrower   38 43          
Restricted cash   $ 184,000 $ 1,788,000          
Number of board of directors | member   4            
Number of board of directors selected by company | member   2            
Number of board of directors selected by counterparty | member   2            
Number of investment committee members selected by company | member   1            
Number of investment committee members selected by counterparty | member   1            
Credit facilities payable   $ 53,000,000 82,082,000          
Assets   141,167,000 146,808,000          
Financial support funded by company and counterparty   84,000,000 84,000,000          
Financial support funded by company for investee   73,500,000 73,500,000          
Investment owned, at cost   127,788,000 140,083,000          
Interest income $ 4,643,000 14,043,000 9,703,000          
Investments sold   6,500,000            
Proceeds from sale of investments   6,300,000            
Financial commitment to investee by company and counterparty, aggregate amount   100,000,000 100,000,000          
Financial commitment to investee, aggregate amount   87,500,000 87,500,000          
Financial commitment to investee by counterparty, aggregate amount   12,500,000 $ 12,500,000          
OCSI Glick JV LLC | Glick JV LLC                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value   $ 50,000,000            
Investment owned, ownership percentage   87.50% 12.50%          
Unfunded commitments   $ 1,600,000 $ 1,600,000          
Investment owned, at cost   50,000,000            
Financial commitment to investee, aggregate amount   $ 8,700,000 8,700,000          
OCSI Glick JV LLC | Glick JV Notes                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Investments at fair value     $ 50,300,000          
Investment owned, ownership percentage   87.50% 12.50%          
Unfunded commitments   $ 12,400,000 $ 12,400,000          
Investment owned, at cost     50,300,000          
Financial commitment to investee, aggregate amount   78,800,000 $ 78,800,000          
OCSI Glick JV LLC | Glick JV Facility | Revolving Credit Facility | Line of Credit                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Borrowing capacity   80,000,000            
Credit facilities payable   $ 53,000,000            
OCSI Glick JV LLC | Glick JV Facility | Revolving Credit Facility | Line of Credit | Until November 3, 2023 | Secured Overnight Financing Rate                
Investment Company, Nonconsolidated Subsidiary [Line Items]                
Debt instrument, basis spread, variable rate   2.00%