Consolidated Statements of Changes in Net Assets (unaudited) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended		11 Months Ended
	Jun. 30, 2023	Apr. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2024
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)						169,594,742
Beginning balance		$ 2,506,145		$ 2,593,640	$ 2,506,145	$ 2,547,878	$ 2,544,500
Repurchases of common stock, net of commission costs (in shares)	(299,794)	(56,130)	(751,544)	0		0	(1,295,678)	(1,300,928)	0
Repurchases of common stock, net of commission costs	$ (3,975)	$ (731)	$ (9,647)		(7,144)		$ (16,791)	$ (16,861)
Issuance of common stock - GBDC 3 Merger				$ 1,526,351		$ 1,526,351
Net increase (decrease) in net assets resulting from operations
Net investment income after taxes				92,722	73,778	262,790	205,647
Net realized gain (loss) on investment transactions				(37,719)	(41,923)	(50,441)	(43,391)
Net change in unrealized appreciation (depreciation) on investment transactions				(45,941)	41,050	(34,280)	(5,558)
(Provision) benefit for taxes on realized gain on investments				0	0	0	(207)
(Provision) benefit for taxes on unrealized appreciation on investments				190	78	517	576
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan				14,763		44,283
Distributions from distributable earnings				(90,363)	(56,066)	(243,455)	(168,858)
Total increase (decrease) common stock, outstanding value				$ 1,460,003	$ 9,773	$ 1,505,765	$ (28,582)
Ending balance (in shares)	169,599,992			264,609,056	169,599,992	264,609,056	169,599,992	264,609,056	264,609,056
Ending balance	$ 2,515,918		$ 2,506,145	$ 4,053,643	$ 2,515,918	$ 4,053,643	$ 2,515,918	$ 4,053,643	$ 4,053,643
Common Stock
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)		170,144,126		171,517,307	170,144,126	169,594,742	170,895,670
Beginning balance		$ 170		$ 172	$ 170	$ 170	$ 171
Repurchases of common stock, net of commission costs (in shares)					(544,134)		(1,295,678)
Repurchases of common stock, net of commission costs							$ (1)
Issuance of common stock - GBDC 3 Merger (in shares)				92,115,308		92,115,308
Issuance of common stock - GBDC 3 Merger				$ 92		$ 92
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan (in shares)				976,441		2,899,006
Stock issued in connection with dividend reinvestment plan				$ 1		$ 3
Total increase (decrease) common stock, outstanding (in shares)				93,091,749	(544,134)	95,014,314	(1,295,678)
Total increase (decrease) common stock, outstanding value				$ 93	$ 0	$ 95	$ (1)
Ending balance (in shares)	169,599,992		170,144,126	264,609,056	169,599,992	264,609,056	169,599,992	264,609,056	264,609,056
Ending balance	$ 170		$ 170	$ 265	$ 170	$ 265	$ 170	$ 265	$ 265
Paid in Capital in Excess of Par
Investment Company, Net Assets [Roll Forward]
Beginning balance		2,667,028		2,676,430	2,667,028	2,646,912	2,676,674
Repurchases of common stock, net of commission costs					(7,144)		(16,790)
Issuance of common stock - GBDC 3 Merger				1,526,259		1,526,259
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan				14,762		44,280
Total increase (decrease) common stock, outstanding value				1,541,021	(7,144)	1,570,539	(16,790)
Ending balance	2,659,884		2,667,028	4,217,451	2,659,884	4,217,451	2,659,884	4,217,451	4,217,451
Distributable Earnings (Losses)
Investment Company, Net Assets [Roll Forward]
Beginning balance		$ (161,053)		(82,962)	(161,053)	(99,204)	(132,345)
Net increase (decrease) in net assets resulting from operations
Net investment income after taxes				92,722	73,778	262,790	205,647
Net realized gain (loss) on investment transactions				(37,719)	(41,923)	(50,441)	(43,391)
Net change in unrealized appreciation (depreciation) on investment transactions				(45,941)	41,050	(34,280)	(5,558)
(Provision) benefit for taxes on realized gain on investments							(207)
(Provision) benefit for taxes on unrealized appreciation on investments				190	78	517	576
Distributions to stockholders:
Distributions from distributable earnings				(90,363)	(56,066)	(243,455)	(168,858)
Total increase (decrease) common stock, outstanding value				(81,111)	16,917	(64,869)	(11,791)
Ending balance	$ (144,136)		$ (161,053)	$ (164,073)	$ (144,136)	$ (164,073)	$ (144,136)	$ (164,073)	$ (164,073)