Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Debt Instrument [Line Items]
Accretion of discounts and amortization of premiums on investments			$ (6,904)	$ (1,748)
Amortization of debt issuance costs	$ 1,677	$ 1,776	3,425	3,443
Cash paid for interest expense			63,401	29,663
Average outstanding balance			3,069,131	2,740,594
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	6,456	1,904	12,260	3,750
Amortization of debt issuance costs	30	104	143	210
Total interest and other debt financing expenses	6,486	2,008	12,403	3,960
Cash paid for interest expense	$ 6,134	$ 1,846	$ 10,669	$ 3,694
Annualized average stated interest rate	6.40%	1.90%	6.00%	1.80%
Average outstanding balance	$ 408,200	$ 408,200	$ 408,200	$ 408,200
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	8,103	2,444	15,398	4,818
Accretion of discounts on notes issued	93	441	544	892
Amortization of debt issuance costs	4	17	21	34
Total interest and other debt financing expenses	8,200	2,902	15,963	5,744
Cash paid for interest expense	$ 7,705	$ 2,368	$ 13,420	$ 4,739
Annualized average stated interest rate	6.00%	1.80%	5.70%	1.80%
Average outstanding balance	$ 546,500	$ 546,500	$ 546,500	$ 546,500
MS Credit Facility II
Debt Instrument [Line Items]
Stated interest expense	0	0	0	4
Facility fees	0	148	0	188
Amortization of debt issuance costs	0	52	0	106
Total interest and other debt financing expenses	0	200	0	298
Cash paid for interest expense	0	150	0	296
Average outstanding balance	0	0	0	0
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense	9,781	2,086	18,268	3,475
Facility fees	413	742	1,140	1,326
Amortization of debt issuance costs	576	486	1,112	874
Total interest and other debt financing expenses	10,770	3,314	20,520	5,675
Cash paid for interest expense	$ 10,957	$ 2,620	$ 19,787	$ 3,539
Annualized average stated interest rate	5.90%	2.00%	5.50%	2.00%
Average outstanding balance	$ 668,980	$ 420,435	$ 664,431	$ 356,773
2024 Notes
Debt Instrument [Line Items]
Stated interest expense	4,219	4,219	8,438	8,307
Accretion of discounts and amortization of premiums on investments	(341)	(341)	(690)	(633)
Amortization of debt issuance costs	490	480	990	951
Total interest and other debt financing expenses	4,368	4,358	8,738	8,625
Cash paid for interest expense	$ 0	$ 0	$ 8,438	$ 6,750
Annualized average stated interest rate	3.40%	3.40%	3.40%	3.40%
Average outstanding balance	$ 500,000	$ 500,000	$ 500,000	$ 492,308
2026 Notes
Debt Instrument [Line Items]
Stated interest expense	3,750	3,750	7,500	7,333
Accretion of discounts on notes issued	131	131	265	254
Amortization of debt issuance costs	368	403	745	794
Total interest and other debt financing expenses	4,249	4,284	8,510	8,381
Cash paid for interest expense	$ 7,500	$ 6,819	$ 7,500	$ 6,819
Annualized average stated interest rate	2.50%	2.50%	2.50%	2.50%
Average outstanding balance	$ 600,000	$ 600,000	$ 600,000	$ 586,813
2027 Notes
Debt Instrument [Line Items]
Stated interest expense	1,794	1,794	3,588	3,588
Accretion of discounts on notes issued	180	181	365	366
Amortization of debt issuance costs	209	234	414	474
Total interest and other debt financing expenses	2,183	2,209	4,367	4,428
Cash paid for interest expense	$ 3,587	$ 3,827	$ 3,587	$ 3,827
Annualized average stated interest rate	2.10%	2.10%	2.10%	2.10%
Average outstanding balance	$ 350,000	$ 350,000	$ 350,000	$ 350,000
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense	0	0	0	0
Cash paid for interest expense	0	0	0	0
Average outstanding balance	$ 0	$ 0	$ 0	$ 0