Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Debt Instrument [Line Items]
Net (gain)/loss related to the fair value hedge		$ 4,557	$ (4,598)
Accretion of discounts and amortization of premiums on notes issued		(8,906)	(10,403)	$ (12,878)
Amortization of debt issuance costs		9,995	9,237	7,380
Cash paid (received) for interest, other debt financing expenses, and facility fees	[1]	280,229	180,066	138,441
Average outstanding balance		4,892,712	3,492,575	3,069,412	$ 2,935,846	$ 2,184,010
2024 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		71,193	0	0
Amortization of debt issuance costs		809	0	0
Total interest and other debt financing expenses		72,002	0	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 55,072	0	0
Average stated interest rate		6.00%
Average outstanding balance		$ 1,184,625	0	0
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		1,839	24,042	26,375
Amortization of debt issuance costs		0	2	161
Total interest and other debt financing expenses		1,839	24,044	26,536
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 5,152	$ 26,430	$ 24,271
Average stated interest rate		7.10%	7.40%	6.50%
Average outstanding balance		$ 25,881	$ 324,648	$ 403,098
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		1,682	28,062	32,923
Accretion of discounts on notes issued		0	0	544
Amortization of debt issuance costs		0	0	21
Total interest and other debt financing expenses		1,682	28,062	33,488
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 4,861	$ 31,909	$ 30,453
Average stated interest rate		6.10%	6.80%	6.10%
Average outstanding balance		$ 27,602	$ 412,682	$ 537,754
GBDC 3 2021 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		2,694	7,181	0
Amortization of debt issuance costs		0	0	0
Total interest and other debt financing expenses		2,694	7,181	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 7,355	$ 2,520	0
Average stated interest rate		6.90%	7.30%
Average outstanding balance		$ 39,189	$ 97,705	0
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		11,364	5,909	0
Amortization of debt issuance costs		0	0	0
Total interest and other debt financing expenses		11,364	5,909	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 14,765	$ 2,508	0
Average stated interest rate		6.60%	7.40%
Average outstanding balance		$ 173,187	$ 79,626	0
GBDC 3 2022-2 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense		3,504	5,923	0
Amortization of debt issuance costs		0	0	0
Total interest and other debt financing expenses		3,504	5,923	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 7,197	$ 2,229	0
Average stated interest rate		7.50%	8.00%
Average outstanding balance		$ 46,849	$ 73,770	0
GBDC 3 DB Credit Facility
Debt Instrument [Line Items]
Stated interest expense		3,820	11,219	0
Facility fees		244	680	0
Accretion of discounts and amortization of premiums on notes issued		0	1	0
Total interest and other debt financing expenses		4,064	11,900	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 11,680	$ 4,284	0
Average stated interest rate		6.70%	7.60%
Average outstanding balance		$ 57,154	$ 148,365	0
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense		69,268	26,777	41,821
Facility fees		2,820	4,389	2,635
Amortization of debt issuance costs		3,945	3,569	2,793
Total interest and other debt financing expenses		76,033	34,735	47,249
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 76,080	$ 27,612	$ 44,667
Average stated interest rate		5.70%	7.10%	6.20%
Average outstanding balance		$ 1,209,252	$ 377,773	$ 678,560
2024 Notes
Debt Instrument [Line Items]
Stated interest expense		0	8,766	16,876
Accretion of discounts and amortization of premiums on notes issued		0	(747)	(1,384)
Amortization of debt issuance costs		0	1,140	1,986
Total interest and other debt financing expenses		0	9,159	17,478
Cash paid (received) for interest, other debt financing expenses, and facility fees		0	$ 16,547	$ 16,876
Average stated interest rate			3.40%	3.40%
Average outstanding balance		0	$ 259,563	$ 500,000
2026 Notes
Debt Instrument [Line Items]
Stated interest expense		15,000	15,000	15,000
Accretion of discounts on notes issued		531	532	531
Amortization of debt issuance costs		1,537	1,635	1,493
Total interest and other debt financing expenses		17,068	17,167	17,024
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 15,000	$ 15,000	$ 15,000
Average stated interest rate		2.50%	2.50%	2.50%
Average outstanding balance		$ 600,000	$ 600,000	$ 600,000
2027 Notes
Debt Instrument [Line Items]
Stated interest expense		7,176	7,176	7,176
Accretion of discounts on notes issued		732	736	733
Amortization of debt issuance costs		840	1,021	926
Total interest and other debt financing expenses		8,748	8,933	8,835
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 7,176	$ 7,176	$ 7,174
Average stated interest rate		2.10%	2.10%	2.10%
Average outstanding balance		$ 350,000	$ 350,000	$ 350,000
2028 Notes
Debt Instrument [Line Items]
Stated interest expense		31,969	26,084	0
Net contractual interest rate swap expense		1,358	3,775	0
Net (gain)/loss related to the fair value hedge		7,398	(7,549)	0
Accretion of discounts on notes issued		904	795	0
Amortization of debt issuance costs		1,216	975	0
Total interest and other debt financing expenses		42,845	24,080	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 27,920	$ 20,005	0
Average stated interest rate		7.40%	8.10%
Average outstanding balance		$ 453,425	$ 370,082	0
2029 Notes
Debt Instrument [Line Items]
Stated interest expense		43,450	24,000	0
Net contractual interest rate swap expense		5,345	6,631	0
Net (gain)/loss related to the fair value hedge		(2,842)	2,951	0
Accretion of discounts on notes issued		1,116	826	0
Amortization of debt issuance costs		1,649	895	0
Total interest and other debt financing expenses		48,718	35,303	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		$ 47,971	$ 23,846	0
Average stated interest rate		6.70%	7.60%
Average outstanding balance		$ 724,110	$ 398,361	0
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense		57	0	0
Cash paid (received) for interest, other debt financing expenses, and facility fees		0	0	0
Average outstanding balance		$ 1,438	$ 0	$ 0

[1] Includes cash paid or received for contractual interest rate swaps, which may have different contractual settlement dates among the (i) fixed interest leg, (ii) floating interest leg and (iii) the debt instrument.