Financial Highlights - Schedule of Investment Company, Financial Highlights (Details) $ / shares in Units, $ in Thousands				3 Months Ended
	Dec. 08, 2023 $ / shares	Dec. 01, 2023 $ / shares	Dec. 09, 2022 $ / shares	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Feb. 06, 2019	Feb. 05, 2019
Investment Company, Financial Highlights [Roll Forward]
Net asset value at beginning of period (in dollars per share)				$ 15.02	$ 14.89
From net investment income - after tax (in dollars per share)	$ (0.37)	$ (0.07)	$ (0.33)
Net investment income - after tax (in dollars per share)				0.49	0.36
Net realized gain (loss) on investment transactions (in dollars per share)				0.01	0.02
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)				(0.05)	(0.23)
Net asset value at end of period (in dollars per share)				15.03	14.71
Per share market value at end of period (in dollars per share)				$ 15.10	$ 13.49
Total return based on market value				5.90%	8.90%
Number of common shares outstanding (in shares) | shares				170,585,795	170,895,670	169,594,742
Ratio of net investment income - after tax to average net assets				13.00%	9.61%
Ratio of total expenses to average net assets				10.10%	9.52%
Ratio of incentive fees to average net assets				0.83%	0.63%
Ratio of excise tax to average net assets				0.02%	0.09%
Ratio of net expenses (without incentive fees) to average net assets				9.27%	8.89%
Total return based on average net asset value				2.96%	1.01%
Total return based on average net asset value - annualized				11.80%	3.99%
Net assets at end of period | $				$ 2,563,918	$ 2,513,685	$ 2,547,878	$ 2,544,500
Average debt outstanding | $				$ 3,078,186	$ 3,081,266
Average debt outstanding per share (in dollars per share)				$ 18.04	$ 18.03
Portfolio Turnover				5.31%	17.97%
Asset coverage ratio				182.44%	180.44%
Asset coverage ratio per unit (in dollars per unit)				$ 1.825	$ 1.804
Minimum asset coverage ratio required								1.50	1.50
2024 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding | $				$ 500,000	$ 500,000
Average market value per unit (in dollars per share)				$ 989,000	$ 959,000
2026 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding | $				$ 600,000	$ 600,000
Average market value per unit (in dollars per share)				$ 892,000	$ 853,000
2027 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding | $				$ 350,000	$ 350,000
Average market value per unit (in dollars per share)				$ 864,000	$ 819,000
2028 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding | $				$ 132,065
Average market value per unit (in dollars per share)				$ 819,000
From net investment income - after tax
Investment Company, Financial Highlights [Roll Forward]
From net investment income - after tax (in dollars per share)				(0.44)	(0.33)
Dividend Reinvestment Plan (DRIP)
Investment Company, Financial Highlights [Roll Forward]
Net increase in net assets as a result of issuance of DRIP shares (in dollars per share)				$ 0	$ 0