v3.26.1
Investments - Schedules of Portfolio Composition by Geographic Region at Amortization Cost and Fair Value (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Jun. 30, 2026
Sep. 30, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 8,377,113 $ 8,759,938
Fair Value $ 8,196,353 [1] $ 8,769,389 [2]
Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 100.00% 100.00%
Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 100.00% 100.00%
Mid-Atlantic    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 1,636,482 $ 1,621,070
Fair Value $ 1,627,518 $ 1,641,053
Mid-Atlantic | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 19.50% 18.50%
Mid-Atlantic | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 19.90% 18.70%
Southeast    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 1,625,942 $ 1,756,509
Fair Value $ 1,578,872 $ 1,741,027
Southeast | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 19.40% 20.00%
Southeast | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 19.30% 19.80%
Midwest    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 1,276,837 $ 1,395,381
Fair Value $ 1,265,938 $ 1,399,325
Midwest | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 15.20% 15.90%
Midwest | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 15.40% 16.00%
West    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 1,179,605 $ 1,345,106
Fair Value $ 1,151,236 $ 1,347,579
West | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 14.10% 15.40%
West | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 14.00% 15.40%
Southwest    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 1,061,323 $ 1,055,464
Fair Value $ 988,312 $ 1,028,584
Southwest | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 12.70% 12.00%
Southwest | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 12.10% 11.70%
Northeast    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 477,924 $ 472,221
Fair Value $ 469,869 $ 472,847
Northeast | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 5.70% 5.40%
Northeast | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 5.70% 5.40%
United Kingdom    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 408,005 $ 399,688
Fair Value $ 405,116 $ 407,437
United Kingdom | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 4.90% 4.60%
United Kingdom | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 4.80% 4.60%
Canada    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 343,820 $ 337,991
Fair Value $ 334,717 $ 332,821
Canada | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 4.10% 3.90%
Canada | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 4.10% 3.80%
Finland    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 81,247 $ 78,934
Fair Value $ 85,753 $ 88,087
Finland | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 1.00% 0.90%
Finland | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 1.00% 1.00%
Germany    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 64,945 $ 81,459
Fair Value $ 64,723 $ 87,711
Germany | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.80% 0.90%
Germany | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.80% 1.00%
Jersey    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 57,547 $ 57,493
Fair Value $ 57,769 $ 59,437
Jersey | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.70% 0.70%
Jersey | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.70% 0.70%
Australia    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 54,015 $ 54,443
Fair Value $ 56,528 $ 54,102
Australia | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.60% 0.60%
Australia | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.70% 0.60%
Sweden    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 36,384 $ 35,269
Fair Value $ 37,450 $ 37,383
Sweden | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.40% 0.40%
Sweden | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.50% 0.40%
France    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 28,192 $ 20,285
Fair Value $ 28,746 $ 22,233
France | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.30% 0.20%
France | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.40% 0.20%
Netherlands    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 17,737 $ 12,835
Fair Value $ 17,492 $ 13,210
Netherlands | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.20% 0.10%
Netherlands | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.20% 0.20%
Israel    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 17,492 $ 17,384
Fair Value $ 17,253 $ 17,757
Israel | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.20% 0.20%
Israel | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.20% 0.20%
Denmark    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 4,870 $ 4,866
Fair Value $ 4,758 $ 4,821
Denmark | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.10% 0.10%
Denmark | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.10% 0.10%
Luxembourg    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 4,746 $ 4,610
Fair Value $ 4,303 $ 4,890
Luxembourg | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.10% 0.10%
Luxembourg | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.10% 0.10%
Spain    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Amortized Cost $ 0 $ 8,930
Fair Value $ 0 $ 9,085
Spain | Amortized Cost | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.00% 0.10%
Spain | Fair Value | Geographic Concentration Risk    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value: 0.00% 0.10%
[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. 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 noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.