v3.26.1
Portfolio Investments - Schedule of Significant Unobservable Inputs for Level 3 Investments (Details)
$ in Thousands
Jun. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 2,741,814 $ 2,847,782
Senior secured debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 2,316,840 2,445,538
Subordinated debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 162,682 90,460
Debt investments in the JVs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 125,828 158,716
Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 2,508,878 2,477,378
Level 3 | Senior secured debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 2,160,672 2,107,306
Level 3 | Senior secured debt | Market Yield | Market Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 2,034,574 $ 1,997,472
Level 3 | Senior secured debt | Market Yield | Market Yield | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.060 0.070
Level 3 | Senior secured debt | Market Yield | Market Yield | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.350 0.360
Level 3 | Senior secured debt | Market Yield | Market Yield | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.113 0.107
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 28,470 $ 51,483
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.3 0.7
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 4.8 5.5
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 1.7 2.7
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 31,786 $ 11,414
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 6.0 0.4
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 11.5 7.3
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 8.6 3.4
Level 3 | Senior secured debt | Transaction Precedent | Transaction Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 25,098 $ 46,937
Level 3 | Senior secured debt | Broker Quotations | Broker Quoted Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 40,744  
Level 3 | Subordinated debt | Market Yield | Market Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 50,122 $ 72,421
Level 3 | Subordinated debt | Market Yield | Market Yield | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.050 0.050
Level 3 | Subordinated debt | Market Yield | Market Yield | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.110 0.120
Level 3 | Subordinated debt | Market Yield | Market Yield | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.091 0.087
Level 3 | Subordinated debt | Market Yield | Revenue Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.8  
Level 3 | Subordinated debt | Market Yield | Revenue Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 8.0  
Level 3 | Subordinated debt | Market Yield | Revenue Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 8.0  
Level 3 | Subordinated debt | Enterprise Value | Revenue Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 44,980  
Level 3 | Subordinated debt | Transaction Precedent | Transaction Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 21,847  
Level 3 | Debt investments in the JVs | Enterprise Value    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 125,828 $ 158,716
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 48,262 $ 58,219
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.0 0.3
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 8.0 5.5
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.4 0.5
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 50,349 $ 72,013
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 0.4 2.3
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 14.3 14.3
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input 8.8 10.3
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value   $ 1,496
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input   1.4
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input   1.6
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value, measurement input   1.5
Level 3 | Preferred & Common Equity | Transaction Precedent | Transaction Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value $ 6,818 $ 7,207