v3.26.1
Fair Value Measurements - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Sep. 30, 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Transferred from Level 2 to Level 3     $ 46,027 $ 7,992  
Transferred from Level 3 to Level 2     63,031 22,872  
Fair value $ 9,616,525 [1]   9,616,525 [1]   $ 8,555,251 [2]
Net change in unrealized appreciation (depreciation) on investment transactions 33,412 $ 14,799 (156,878) 17,809  
Level 2          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Fair value 1,276,609   1,276,609   1,750,395
Level 3          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Fair value 8,339,916   8,339,916   6,804,856
Net change in unrealized appreciation (depreciation) on investment transactions 6,231 $ 45,227 (117,669) 51,099  
Debt Investments          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Transferred from Level 2 to Level 3     46,027 7,992  
Transferred from Level 3 to Level 2     63,031 $ 22,872  
Fair value 9,516,858   9,516,858   8,490,414
Debt Investments | Level 2          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Fair value 1,276,609   1,276,609   1,750,395
Debt Investments | Level 3          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Fair value $ 8,240,249   $ 8,240,249   $ 6,740,019
[1] The fair values of investments were valued using significant unobservable inputs, unless otherwise noted. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.
[2] The fair values of investments were valued using significant unobservable inputs, unless otherwise noted. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.