v3.23.3
Portfolio Investments - Roll-Forward in the Changes in Fair Value (Details) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning balance $ 2,169,475 $ 2,161,581
Purchases (a) 1,131,251 520,510
Sales and repayments (665,727) (512,535)
Transfers in 59,168 49,843
Transfers out (40,093) (22,908)
Capitalized PIK interest income 15,644 23,168
Accretion of OID 17,771 26,482
Fair value, ending balance $ 2,619,912 $ 2,169,475
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax
Net unrealized appreciation (depreciation) relating to Level 3 investments still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations $ (63,990) $ (72,913)
Net unrealized appreciation (depreciation)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net unrealized appreciation (depreciation) Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax
Total gains or losses included in earnings $ (48,971) $ (81,863)
Net realized gains (losses)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Realized gain (loss) Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax
Total gains or losses included in earnings $ (18,606) $ 5,197
Senior secured debt    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning balance 1,910,606 1,878,536
Purchases (a) 1,080,654 490,081
Sales and repayments (650,437) (476,813)
Transfers in 19,075 49,843
Transfers out (40,093) (17,070)
Capitalized PIK interest income 15,016 22,855
Accretion of OID 16,322 24,422
Fair value, ending balance 2,292,691 1,910,606
Net unrealized appreciation (depreciation) relating to Level 3 investments still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations (48,680) (53,013)
Senior secured debt | Net unrealized appreciation (depreciation)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings (37,253) (67,455)
Senior secured debt | Net realized gains (losses)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings (21,199) 6,207
Subordinated debt    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning balance 159,388 176,317
Purchases (a) 30,690 7,960
Sales and repayments (1,396) (22,525)
Transfers in 0 0
Transfers out 0 0
Capitalized PIK interest income 628 313
Accretion of OID 1,449 2,060
Fair value, ending balance 189,724 159,388
Net unrealized appreciation (depreciation) relating to Level 3 investments still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations (1,035) (4,885)
Subordinated debt | Net unrealized appreciation (depreciation)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings (1,035) (4,737)
Subordinated debt | Net realized gains (losses)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings 0 0
Preferred equity    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning balance 79,523 63,565
Purchases (a) 14,296 19,662
Sales and repayments 0 (163)
Transfers in 0 0
Transfers out 0 0
Capitalized PIK interest income 0 0
Accretion of OID 0 0
Fair value, ending balance 86,057 79,523
Net unrealized appreciation (depreciation) relating to Level 3 investments still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations (7,762) (3,264)
Preferred equity | Net unrealized appreciation (depreciation)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings (7,762) (3,029)
Preferred equity | Net realized gains (losses)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings 0 (512)
Common equity and warrants    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning balance 19,958 43,163
Purchases (a) 5,611 2,807
Sales and repayments (13,894) (13,034)
Transfers in 40,093 0
Transfers out 0 (5,838)
Capitalized PIK interest income 0 0
Accretion of OID 0 0
Fair value, ending balance 51,440 19,958
Net unrealized appreciation (depreciation) relating to Level 3 investments still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations (6,513) (11,751)
Common equity and warrants | Net unrealized appreciation (depreciation)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings (2,921) (6,642)
Common equity and warrants | Net realized gains (losses)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains or losses included in earnings $ 2,593 $ (498)