Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Cash flows from operating activities
Net increase (decrease) in net assets resulting from operations	$ 258,631	$ 153,440	$ 340,280
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	7,380	7,337	10,203
Accretion of discounts and amortization of premiums on investments	(12,878)	(9,047)	9,394
Accretion of discounts and amortization of premiums on issued debt securities	424	1,715	2,129
Net realized (gain) loss on investments	46,496	(18,925)	(13,006)
Net realized (gain) loss on foreign currency transactions	328	(371)	5,101
Net realized (gain) loss on forward currency contracts	(2,711)	(1,080)	0
Net change in unrealized (appreciation) depreciation on investments	(14,267)	56,806	(161,867)
Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies	(16,680)	37,335	(2,225)
Net change in unrealized (appreciation) depreciation on forward currency contracts	17,392	(32,243)	(1,154)
Proceeds from (fundings of) revolving loans, net	(381)	(1,812)	12,170
Fundings of investments	(675,341)	(1,883,080)	(2,082,127)
Proceeds from principal payments and sales of portfolio investments	670,188	1,260,787	1,593,478
Proceeds from settlements of forward currency contracts	2,711	1,080	0
Payment-in-kind interest capitalized	(38,844)	(21,506)	(16,092)
Non-cash dividends capitalized	(14,901)	0	0
Proceeds from non-cash dividends	10	0	0
Changes in operating assets and liabilities:
Interest receivable	(37,260)	(2,533)	(998)
Cash collateral held at broker for forward currency contracts	0	6,960	(3,640)
Receivable from investments sold	0	97	162
Other assets	(2,470)	(910)	524
Interest payable	4,365	7,868	4,641
Management and incentive fees payable	1,847	21,183	(5,100)
Payable for investments purchased	0	(294)	294
Accrued trustee fees	106	225	0
Accounts payable and other liabilities	1,225	505	1,785
Net cash provided by (used in) operating activities	195,370	(416,463)	(306,048)
Cash flows from financing activities
Borrowings on debt	652,583	1,292,672	3,358,842
Repayments of debt	(627,094)	(741,211)	(2,816,054)
Capitalized debt issuance costs	(5,782)	(6,698)	(22,157)
Purchases of common stock (Note 12)	(16,861)	0	0
Distributions paid	(191,465)	(155,208)	(139,122)
Purchases of common stock for dividend reinvestment plan	(46,927)	(36,424)	(14,659)
Net cash provided by (used in) financing activities	(235,546)	353,131	366,850
Net change in cash and cash equivalents, foreign currencies, restricted cash and cash equivalents	(40,176)	(63,332)	60,802
Effect of foreign currency exchange rates	(171)	(458)	(889)
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents, beginning of period	180,553	244,343	184,430
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents, end of period	140,206	180,553	244,343
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	138,441	72,458	48,766
Distributions declared for the period	238,392	204,806	194,852
Supplemental disclosure of non-cash financing activities:
Stock issued in connection with dividend reinvestment plan	0	13,174	41,071
Proceeds from issuance of Class A-2-R GCIC 2018 Notes	0	0	38,500
Redemptions of Class A-2 GCIC 2018 Notes	0	0	(38,500)
Reconciliation of cash and cash equivalents, foreign currencies and restricted cash and cash equivalents and restricted foreign currencies
Cash and cash equivalents	65,617	117,290
Foreign currencies (cost of $4,540 and $7,021, respectively)	4,208	6,847
Restricted cash and cash equivalents	70,381	56,416
Total cash and cash equivalents, foreign currencies and restricted cash and cash equivalents shown in the Consolidated Statements of Cash Flows	$ 140,206	$ 180,553	$ 244,343