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Significant Accounting Policies - Additional Information (Details)
 12 Months Ended
Sep. 30, 2022
USD ($)
Portfolio
Sep. 30, 2021
USD ($)
Portfolio
Sep. 30, 2020
USD ($)
Significant Accounting Policies [Line Items]      
Provision for taxes $ 400,000 $ 400,000 $ 400,000
Provision for taxes on net unrealized gain on investments 4,600,000 0  
Deferred tax liability 4,600,000 0  
Provision for taxes on net realized gain on investments 1,200,000    
Prepaid tax asset 1,200,000    
Tax accruals relating to uncertain tax positions 0 0 0
Amounts accrued for interest or penalties $ 0 $ 0 $ 0
Past Due 30 Days or More      
Significant Accounting Policies [Line Items]      
Financing receivable, nonaccrual, percent past due on cost basis 0.90% 2.70%  
Financing receivable, nonaccrual, percent past due on fair value basis 0.00% 2.60%  
Number of portfolio on nonaccrual | Portfolio 2 2