Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended			12 Months Ended
		Jun. 30, 2024		Jun. 30, 2023	Jun. 30, 2024		Jun. 30, 2023	Sep. 30, 2023
Cash flows from operating activities
Net increase (decrease) in net assets resulting from operations		$ 9,252		$ 72,983	$ 178,586		$ 157,067
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		2,214		1,937	6,869		5,362
Accretion of discounts and amortization of premiums on investments					1,267		454
Accretion of discounts and amortization of premiums on issued debt securities					(10,996)		(9,994)
Net realized (gain) loss on investments					49,505		46,555
Net realized (gain) loss on foreign currency transactions		6,206		(1,141)	1,560		28
Net realized (gain) loss on forward currency contracts		(624)		(2,021)	(624)		(3,192)
Net change in unrealized (appreciation) depreciation on investments					35,609		5,089
Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies		(3,709)		(3,399)	260		(21,857)
Proceeds from (fundings of) revolving loans, net					(4,658)		(2,449)
Fundings of investments					(554,521)		(583,847)
Proceeds from principal payments and sales of portfolio investments					867,233		550,843
Proceeds from settlements of forward currency contracts					1,123		3,192
Payment-in-kind interest capitalized		(11,245)		(10,405)	(36,551)		(27,660)
Non-cash dividends capitalized					(13,890)		(10,767)
Proceeds from non-cash dividends					47		0
Cash acquired in GBDC 3 Merger					53,885		0
Changes in operating assets and liabilities:
Cash collateral held at broker for forward currency contracts					(11,670)		0
Interest receivable					8,374		(9,139)
Receivable for investments sold					0		0
Other assets					(20,191)		340
Interest payable					23,750		5,791
Management and incentive fees payable					(29,853)		4,424
Accrued trustee fees					155		(72)
Payable for investments purchased					0		250
Accounts payable and other liabilities					(2,730)		(1,497)
Net cash provided by (used in) operating activities					543,942		131,247
Cash flows from financing activities
Borrowings on debt					1,966,609		513,927
Repayments of debt					(1,971,861)		(463,765)
Capitalized debt issuance costs					(13,488)		(5,368)
Purchases of common stock (Note 11)					0		(16,791)
Distributions paid	[1]				(199,173)		(135,311)
Purchases of common stock for dividend reinvestment plan					0		(33,547)
Net cash provided by (used in) financing activities					(217,913)		(140,855)
Net change in cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies					326,029		(9,608)
Effect of foreign currency exchange rates					(1,521)		93
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, beginning of period					140,206	[2]	180,553	$ 180,553
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, end of period		464,714	[2]	171,038	464,714	[2]	171,038	140,206	[2]
Supplemental disclosure of cash flow information:
Cash paid during the period for interest					110,623		98,381
Distributions declared for the period		243,455		168,858	243,455		168,858
Supplemental disclosure of non-cash financing activities:
Stock issued in connection with dividend reinvestment plan					44,282		0
Noncash assets acquired:
Investments, at cost	[3]				2,623,381		0
Interest receivable	[3]				32,204		0
Unrealized appreciation on forward currency contracts	[3]				6,334		0
Receivable for investments sold	[3]				8,922		0
Other assets	[3]				154		0
Total noncash assets purchased	[3]				2,670,995		0
Liabilities assumed:
Debt	[3]				1,211,174		0
Interest payable	[3]				12,671		0
Distributions payable	[3]				9,516		0
Management and incentive fees payable	[3]				8,921		0
Accrued trustee fees	[3]				8		0
Accounts payable and other liabilities	[3]				4,040		0
Total liabilities assumed	[3]				1,246,330		0
Issuance of common stock	[3]				1,526,351		0
Merger costs capitalized into purchase price	[3]				3,867		0
Reconciliation of cash and cash equivalents, foreign currencies and restricted cash and cash equivalents and restricted foreign currencies
Cash and cash equivalents		141,186			141,186			65,617
Foreign currencies (cost of $6,381 and $4,540, respectively)		6,327			6,327			4,208
Restricted cash and cash equivalents		316,484			316,484			70,381
Restricted foreign currencies (cost of $717 and $0, respectively)		717		0	717		0	0
Total cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies shown in the Consolidated Statements of Cash Flows(3)		464,714	[2]	171,038	464,714	[2]	171,038	$ 140,206	[2]
Interest rate swaps
Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities:
Net change in unrealized (appreciation) depreciation on derivatives					2,992		0
Forward currency contracts
Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities:
Net realized (gain) loss on forward currency contracts		(624)		(2,021)	(624)		(3,192)
Net change in unrealized (appreciation) depreciation on derivatives		$ (2,972)		$ 5,431	$ (1,589)		$ 22,326

[1] Includes payment of $9,516 distribution payable to GBDC 3 shareholders that was assumed in the GBDC 3 Merger (defined in Note 2).

[2] See “Note 2. Significant Accounting Policies and Recent Accounting Updates” for a description of cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies.

[3] Refer to Note 13 for more information related to our acquisition of GBDC 3.