v3.23.3
Borrowings - Narrative (Details) - USD ($)
 8 Months Ended 12 Months Ended
Aug. 15, 2023
May 18, 2021
Feb. 25, 2020
Sep. 30, 2023
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Line of Credit Facility [Line Items]              
Borrowings outstanding       $ 710,000,000 $ 710,000,000 $ 700,000,000  
Interest expense         111,642,000 46,929,000 $ 30,518,000
Fair Value       1,588,201,000 $ 1,588,201,000 $ 1,277,105,000  
Interest rate swap              
Line of Credit Facility [Line Items]              
Interest rate           2.70%  
Floating interest rate           1.658%  
Notional amount           $ 350,000,000  
Interest rate swap | Designated as Hedging Instrument              
Line of Credit Facility [Line Items]              
Interest rate   2.70%          
Notional amount   $ 350,000,000          
Secured Overnight Financing Rate              
Line of Credit Facility [Line Items]              
Interest rate         2.00%    
Secured Overnight Financing Rate | Minimum              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         0.11448%    
Secured Overnight Financing Rate | Maximum              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         0.26161%    
London Interbank Offered Rate (LIBOR) | Interest rate swap | Designated as Hedging Instrument              
Line of Credit Facility [Line Items]              
Floating interest rate   1.658%          
Base Rate              
Line of Credit Facility [Line Items]              
Interest rate         1.00%    
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Interest rate swap | Designated as Hedging Instrument              
Line of Credit Facility [Line Items]              
Floating interest rate   0.26161%          
Citibank Facility              
Line of Credit Facility [Line Items]              
Interest expense         $ 600,000    
2025 Notes              
Line of Credit Facility [Line Items]              
Principal amount     $ 300,000,000 300,000,000.0 300,000,000.0 300,000,000.0  
Net proceeds     293,800,000        
Unaccreted discount     2,500,000 $ 700,000 $ 700,000 1,200,000  
Underwriting commissions     3,000,000        
Offering costs     $ 700,000        
Interest rate, percentage     3.50% 3.50% 3.50%    
Redemption price     100.00%        
2027 Notes              
Line of Credit Facility [Line Items]              
Principal amount   $ 350,000,000   $ 350,000,000.0 $ 350,000,000.0 350,000,000.0  
Net proceeds   344,800,000          
Unaccreted discount   1,000,000   $ 600,000 $ 600,000 700,000  
Underwriting commissions   3,500,000          
Offering costs   $ 700,000          
Interest rate, percentage   2.70%   2.70% 2.70%    
Redemption price   100.00%          
2029 Notes              
Line of Credit Facility [Line Items]              
Principal amount $ 300,000,000     $ 300,000,000.0 $ 300,000,000.0    
Net proceeds 292,900,000            
Unaccreted discount 3,500,000     $ 3,400,000 $ 3,400,000    
Underwriting commissions 3,000,000            
Offering costs $ 600,000            
Interest rate, percentage 7.10%     7.10% 7.10%    
Redemption price 100.00%            
2029 Notes | Interest rate swap | Designated as Hedging Instrument              
Line of Credit Facility [Line Items]              
Interest rate   7.10%          
Notional amount   $ 300,000,000          
2029 Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Interest rate swap | Designated as Hedging Instrument              
Line of Credit Facility [Line Items]              
Floating interest rate   3.1255%          
Line of Credit | Revolving Credit Facility | Syndicated Facility              
Line of Credit Facility [Line Items]              
Borrowing capacity       $ 1,218,000,000 $ 1,218,000,000    
Accordion feature, increase limit       1,250,000,000 1,250,000,000    
Borrowing capacity subject to expiration       $ 1,035,000,000.000 $ 1,035,000,000.000    
Debt instrument, covenant, total assets to total indebtedness, ratio, minimum threshold       1.50 1.50    
Debt instrument, covenant, consolidated EBITDA to consolidated interest expense, ratio, minimum threshold       2.25 2.25    
Borrowings outstanding       $ 430,000,000 $ 430,000,000 540,000,000  
Fair value       $ 430,000,000 $ 430,000,000 $ 540,000,000  
Weighted average interest rate       6.792% 6.792% 2.876% 2.197%
Interest expense inclusive of fees         $ 50,000,000 $ 19,500,000 $ 13,800,000
Line of Credit | Revolving Credit Facility | Citibank Facility              
Line of Credit Facility [Line Items]              
Borrowings outstanding           160,000,000  
Fair value           $ 160,000,000  
Weighted average interest rate       6.781% 6.781% 3.179% 2.086%
Interest expense inclusive of fees         $ 8,000,000 $ 5,800,000 $ 1,900,000
Line of Credit | Revolving Credit Facility | OSI2 Citibank Facility              
Line of Credit Facility [Line Items]              
Borrowing capacity       $ 400,000,000 400,000,000    
Borrowings outstanding       $ 280,000,000 $ 280,000,000    
Weighted average interest rate       7.666% 7.666%    
Interest expense inclusive of fees       $ 14,600,000      
Unused capacity, commitment fee         0.50%    
Commitment fee, unused capacity threshold percentage       30.00% 30.00%    
Fair Value       $ 280,000,000 $ 280,000,000    
Line of Credit | Revolving Credit Facility | OSI2 Citibank Facility | Secured Overnight Financing Rate | After Reinvestment Period              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         4.00%    
Line of Credit | Revolving Credit Facility | OSI2 Citibank Facility | Secured Overnight Financing Rate | Minimum              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         2.50%    
Line of Credit | Revolving Credit Facility | OSI2 Citibank Facility, Syndicated Loans | Secured Overnight Financing Rate              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         2.00%    
Line of Credit | Revolving Credit Facility | OSI2 Citibank Facility, All Other Loans | Secured Overnight Financing Rate              
Line of Credit Facility [Line Items]              
Debt instrument, basis spread, variable rate         2.75%