Financial Highlights - Schedule of Investment Company, Financial Highlights (Details) - USD ($) $ / shares in Units, $ in Thousands													12 Months Ended
	Sep. 29, 2022	Jun. 29, 2022	Mar. 29, 2022	Dec. 30, 2021	Sep. 29, 2021	Jun. 29, 2021	Mar. 30, 2021	Dec. 30, 2020	Sep. 29, 2020	Jun. 29, 2020	Mar. 27, 2020	Dec. 30, 2019	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Feb. 06, 2019	Feb. 05, 2019
Investment Company, Financial Highlights [Roll Forward]
Net asset value at beginning of period (in dollars per share)													$ 15.19	$ 14.33	$ 16.76	$ 16.10	$ 16.08
Net increase (decrease) in net assets as a result of issuance of DRIP shares													0	0	(1.13)	3.17	0
Distribution declared (in dollars per share)	$ (0.30)	$ (0.30)	$ (0.30)	$ (0.30)	$ (0.29)	$ (0.29)	$ (0.29)	$ (0.29)	$ (0.29)	$ (0.29)	$ (0.33)	$ (0.46)
Net investment income (in dollars per share)													1.15	0.99	0.94	1.36	1.27
Net realized gain (loss) on investment transactions (in dollars per share)													0.12	0.05	(0.12)	(0.07)	0.29
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)													(0.37)	0.98	(0.76)	(2.41)	(0.19)
Net asset value at ending of period (in dollars per share)													14.89	15.19	14.33	16.76	16.10
Per share market value at end of period (in dollars per share)													$ 12.39	$ 15.81	$ 13.24	$ 18.84	$ 18.75
Total return based on market value													(14.80%)	28.90%	(22.81%)	8.80%	7.65%
Number of common shares outstanding (in shares)													170,895,670	170,028,584	167,259,511	132,658,200	60,165,454
Ratio of net investment income to average net assets													7.53%	6.73%	6.22%	8.41%	7.88%
Ratio of total expenses (without waiver) to average net assets													7.43%	5.78%	7.15%	8.42%	7.89%
Ratio of management fee waiver to average net assets													(0.07%)	(0.16%)	0.00%	0.00%	0.00%
Ratio of incentive fees to average net assets													0.68%	0.13%	0.62%	0.87%	1.36%
Ratio of net expenses (without incentive fees) to average net assets													6.68%	5.49%	6.53%	7.55%	6.53%
Total return based on average net asset value													5.89%	13.70%	2.45%	(1.81%)	8.50%
Net assets at end of period													$ 2,544,500	$ 2,582,692	$ 2,396,193	$ 2,222,854	$ 968,854
Average debt outstanding													$ 2,935,846	$ 2,184,010	$ 2,200,950	$ 1,050,155	$ 822,823
Average debt outstanding per share (in dollars per share)													$ 17,180	$ 12.84	$ 13.16	$ 7.92	$ 13.68
Portfolio turnover													23.56%	35.58%	14.87%	17.47%	31.91%
Asset coverage ratio													181.70%	200.04%	232.15%	220.31%	269.51%	150.00%	200.00%
Asset coverage ratio per unit (in dollars per unit)													$ 1,817,000	$ 2,000,000	$ 2,321,000	$ 2,203,000	$ 2,695,000
2024 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding													$ 496,164	$ 398,904	$ 0
Average market value per unit (in dollars per share)													$ 996,000	$ 1,034,000
2026 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding													$ 593,425	$ 240,000	0
Average market value per unit (in dollars per share)													$ 917,000	$ 1,004,000
2027 Notes
Investment Company, Financial Highlights [Roll Forward]
Average debt outstanding													$ 350,000	$ 56,575	$ 0
Average market value per unit (in dollars per share)													$ 888,000	$ 990,000
From net investment income
Investment Company, Financial Highlights [Roll Forward]
Distribution declared (in dollars per share)													(1.20)	(1.13)	$ (1.29)	(1.27)	(1.31)
From capital gains
Investment Company, Financial Highlights [Roll Forward]
Distribution declared (in dollars per share)													0	(0.03)	(0.04)	(0.13)	(0.05)
From return of capital
Investment Company, Financial Highlights [Roll Forward]
Distribution declared (in dollars per share)													0	0	(0.04)	0	0
Dividend Reinvestment Plan (DRIP)
Investment Company, Financial Highlights [Roll Forward]
Net increase (decrease) in net assets as a result of issuance of DRIP shares													$ 0.00	$ 0.00	$ 0.01	$ 0.01	$ 0.01