Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net increase (decrease) in net assets resulting from operations		$ 153,440	$ 340,280	$ 54,872
Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities:
Amortization of deferred debt issuance costs		7,337	10,203	3,534
Accretion of discounts and amortization of premiums on investments		(9,047)	9,394	23,483
Accretion of discounts and amortization of premiums on issued debt securities		1,715	2,129	1,355
Net realized (gain) loss on investments		(19,549)	(13,324)	18,680
Net realized (gain) loss on forward currency contracts		(1,080)	0	0
Net realized (gain) loss on foreign currency transactions		253	5,419	(20)
Net change in unrealized (appreciation) depreciation on investments		56,806	(160,175)	61,850
Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies		37,335	(3,917)	2,728
Net change in unrealized (appreciation) depreciation on forward currency contracts		(32,243)	(1,154)	949
Proceeds from (fundings of) revolving loans, net		(1,812)	12,170	(9,205)
Fundings of investments		(1,883,080)	(2,082,127)	(643,182)
Proceeds from principal payments and sales of portfolio investments		1,260,787	1,593,478	706,044
Proceeds from settlements of forward currency contracts		1,080	0	0
PIK interest		(21,506)	(16,092)	(10,956)
Purchase of SLF and GCIC SLF minority interests, net of cash acquired (Note 1)(1)	[1]	0	0	4,944
Changes in operating assets and liabilities:
Interest receivable		(2,533)	(998)	(3)
Cash collateral held at broker for forward currency contracts		6,960	(3,640)	(2,720)
Receivable from investments sold		97	162	(259)
Other assets		(910)	524	(450)
Interest payable		7,868	4,641	(5,761)
Management and incentive fees payable		21,183	(5,100)	4,463
Payable for investments purchased		(294)	294	0
Accounts payable and other liabilities		505	1,785	(22,455)
Accrued trustee fees		225	0	(207)
Net cash provided by (used in) operating activities		(416,463)	(306,048)	187,684
Cash flows from financing activities
Borrowings on debt		1,292,672	3,358,842	1,053,567
Repayments of debt		(741,211)	(2,816,054)	(1,255,103)
Capitalized debt issuance costs		(6,698)	(22,157)	(4,491)
Proceeds from other short-term borrowings		0	0	64,769
Repayments on other short-term borrowings		0	0	(65,017)
Net proceeds from issuance of common stock (Note 12)		0	0	300,427
Distributions paid		(155,208)	(139,122)	(136,426)
Purchases of common stock under dividend reinvestment plan		(36,424)	(14,659)	(45,534)
Net cash provided by (used in) financing activities		353,131	366,850	(87,808)
Net change in cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies		(63,332)	60,802	99,876
Effect of foreign currency exchange rates		(458)	(889)	346
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, beginning of period		244,343	184,430	84,208
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, end of period		180,553	244,343	184,430
Supplemental disclosure of cash flow information:
Cash paid during the period for interest		72,458	48,766	74,933
Distributions declared during the period		204,806	194,852	202,190
Supplemental disclosure of non-cash operating and financing activities:
Stock issued in connection with dividend reinvestment plan		13,174	41,071	20,230
Noncash assets acquired in consolidation of SLF and GCIC SLF (Note 1)		0	0	185,101
Noncash liabilities assumed in consolidation of SLF and GCIC SLF (Note 1)		0	0	(85,236)
Dissolution of existing SLF and GCIC SLF LLC equity interests		0	0	(119,077)
Proceeds from issuance of Class A-2-R GCIC 2018 Notes		0	38,500	0
Redemptions of Class A-2 GCIC 2018 Notes		0	(38,500)	0
Reconciliation of cash and cash equivalents, foreign currencies and restricted cash and cash equivalents and restricted foreign currencies
Cash and cash equivalents		117,290	175,593
Foreign currencies (cost of $7,021 and $5,145, respectively)		6,847	5,497
Restricted cash and cash equivalents		56,416	61,824
Restricted foreign currencies (cost of $0 and $1,442, respectively)		0	1,429
Total cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies shown in the Consolidated Statements of Cash Flows		180,553	244,343	$ 184,430
Foreign currencies, cost		7,021	5,145
Restricted foreign currencies, cost		$ 0	$ 1,442

[1] Represents $17,011 paid in cash to RGA and Aurora (as defined in Note 1), net of cash acquired due to the consolidation of SLF and GCIC SLF of $21,955.