Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2025		Sep. 30, 2024		Sep. 30, 2023
Cash flows from operating activities
Net increase (decrease) in net assets resulting from operations		$ 376,646		$ 273,785		$ 258,631
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		9,995		9,237		7,380
Accretion of discounts and amortization of premiums on investments		(8,906)		(10,403)		(12,878)
Accretion of discounts and amortization of premiums on issued debt securities		3,283		2,143		424
Net realized (gain) loss on investments		72,214		81,577		46,496
Net realized (gain) loss on foreign currency transactions		4,334		1,969		328
Net realized (gain) loss on forward currency contracts		(4,168)		(3,646)		(2,711)
Net realized (gain) loss on extinguishment of debt		48		0		0
Net change in unrealized (appreciation) depreciation on investments		(50,589)		32,048		(14,267)
Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies		(9,185)		(21,688)		(16,680)
Proceeds from (fundings of) revolving loans, net		(6,253)		(2,596)		(381)
Fundings of investments		(1,952,573)		(1,299,933)		(675,341)
Proceeds from principal payments and sales of portfolio investments		1,516,777		1,262,884		670,188
Proceeds from settlements of forward currency contracts		6,242		7,906		2,711
Payment-in-kind interest capitalized		(55,390)		(52,552)		(38,844)
Non-cash dividends capitalized		(30,707)		(21,264)		(14,901)
Proceeds from non-cash dividends		6,436		47		10
Cash acquired in GBDC 3 Merger		0		53,885		0
Changes in operating assets and liabilities:
Cash collateral held at broker for derivatives		(4,240)		(650)		0
Interest receivable		6,005		16,222		(37,260)
Receivable for investments		(3,728)		8,922		0
Other assets		(3,072)		(4,674)		(2,470)
Interest payable		(7,447)		8,281		4,365
Management and income incentive fees payable		7,265		(10,579)		1,847
Accrued trustee fees		861		(161)		106
Accounts payable and other liabilities	[1]	(540)		(1,259)		1,225
Net cash provided by (used in) operating activities		(113,593)		343,914		195,370
Cash flows from financing activities
Borrowings on debt		4,802,191		2,928,615		652,583
Repayments of debt		(4,514,533)		(2,690,866)		(627,094)
Capitalized debt issuance costs		(10,639)		(18,985)		(5,782)
Net proceeds from issuance of common stock (Note 13)		37,434		0		0
Purchases of common stock (Note 13)		(40,631)		(4,809)		(16,861)
Distributions paid		(368,208)		(338,192)		(191,465)
Purchases of common stock for dividend reinvestment plan		(37,203)		0		(46,927)
Net cash provided by (used in) financing activities		(131,589)		(124,237)		(235,546)
Net change in cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies		(245,182)		219,677		(40,176)
Effect of foreign currency exchange rates		(1,927)		(331)		(171)
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, beginning of period		359,552	[2]	140,206		180,553
Cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies, end of period		112,443	[2]	359,552	[2]	140,206
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	[3]	280,229		180,066		138,441
Distributions declared for the period		439,115		372,959		238,392
Acquisition of subsidiaries
Stock issued in connection with dividend reinvestment plan		33,704		44,283		0
Noncash assets acquired:
Investments, at cost	[4]	0		2,623,381		0
Interest receivable	[4]	0		32,204		0
Unrealized appreciation on forward currency contracts	[4]	0		6,334		0
Receivable for investments sold	[4]	0		8,922		0
Other assets	[4]	0		154		0
Total noncash assets purchased	[4]	0		2,670,995		0
Liabilities assumed:
Debt	[4]	0		1,211,174		0
Interest payable	[4]	0		12,671		0
Distributions payable	[4]	0		9,516		0
Management and income incentive fees payable	[4]	0		8,921		0
Accrued trustee fees	[4]	0		8
Accounts payable and other liabilities	[4]	0		4,040		0
Total liabilities assumed	[4]	0		1,246,330		0
Issuance of common stock	[4]	0		1,526,351		0
Merger costs capitalized into purchase price	[4]	0		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		11,935		123,120
Foreign currencies (cost of $11,685 and $7,973, respectively)		11,681		8,044
Restricted cash and cash equivalents		88,827		227,152
Restricted foreign currencies (cost of $0 and $1,219, respectively)		0		1,236
Total cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies shown in the Consolidated Statements of Cash Flows		112,443	[2]	359,552	[2]	140,206
Interest Rate Swap
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		4,557		(4,598)		0
Foreign exchange
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		(4,168)		(3,646)		(2,711)
Net change in unrealized (appreciation) depreciation on derivatives		$ 8,542		$ 19,011		$ 17,392

[1] (1) September 30, 2024 balance includes payment of $9,516 distribution payable to former GBDC 3 stockholders that was assumed in the GBDC 3 Merger (defined in Note 1).

[2] See Note 2 for a description of cash and cash equivalents, foreign currencies, restricted cash and cash equivalents and restricted foreign currencies.

[3] 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.

[4] Refer to Note 15 for more information related to our acquisition of GBDC 3.