INVESTMENTS - Fair Value Level 3 (Details) $ in Thousands	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 1,363,048	$ 1,349,121
Senior debt and 1st lien notes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	982,334	985,158
Subordinated debt and 2nd lien notes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	107,913	106,894
Equity shares
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	207,185	192,641
Equity warrants
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	1,147	1,043
Structured products
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	27,744	26,173
Level 3
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	1,261,653	1,254,451
Level 3 | Senior debt and 1st lien notes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	946,150	949,079
Level 3 | Senior debt and 1st lien notes | Yield Analysis | Market Yield
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 844,660	$ 844,635
Level 3 | Senior debt and 1st lien notes | Yield Analysis | Market Yield | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.061	0.078
Level 3 | Senior debt and 1st lien notes | Yield Analysis | Market Yield | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.270	0.196
Level 3 | Senior debt and 1st lien notes | Yield Analysis | Market Yield | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	0.109	0.112
Level 3 | Senior debt and 1st lien notes | Market Approach | Adjusted EBITDA Multiple
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 10,163	$ 7,247
Level 3 | Senior debt and 1st lien notes | Market Approach | Adjusted EBITDA Multiple | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	1.2	1.1
Level 3 | Senior debt and 1st lien notes | Market Approach | Adjusted EBITDA Multiple | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	12.5	12.5
Level 3 | Senior debt and 1st lien notes | Market Approach | Adjusted EBITDA Multiple | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	9.3	3.5
Level 3 | Senior debt and 1st lien notes | Recent Transaction | Transaction Price
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 79,471	$ 84,483
Level 3 | Senior debt and 1st lien notes | Recent Transaction | Transaction Price | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.956	0.970
Level 3 | Senior debt and 1st lien notes | Recent Transaction | Transaction Price | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	1.000	1.000
Level 3 | Senior debt and 1st lien notes | Recent Transaction | Transaction Price | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	0.980	0.979
Level 3 | Senior debt and 1st lien notes | Valuation, Unadjusted Prices From Independent Pricing Services And Independent Indicative Broker Quotes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 11,856	$ 12,714
Level 3 | Subordinated debt and 2nd lien notes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	105,067	96,851
Level 3 | Subordinated debt and 2nd lien notes | Yield Analysis | Market Yield
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 87,205	$ 85,803
Level 3 | Subordinated debt and 2nd lien notes | Yield Analysis | Market Yield | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.090	0.085
Level 3 | Subordinated debt and 2nd lien notes | Yield Analysis | Market Yield | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.177	0.189
Level 3 | Subordinated debt and 2nd lien notes | Yield Analysis | Market Yield | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	0.127	0.131
Level 3 | Subordinated debt and 2nd lien notes | Market Approach | Adjusted EBITDA Multiple
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 10,166	$ 4,986
Measurement input		11.0
Level 3 | Subordinated debt and 2nd lien notes | Market Approach | Adjusted EBITDA Multiple | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	6.0
Level 3 | Subordinated debt and 2nd lien notes | Market Approach | Adjusted EBITDA Multiple | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	11.0
Level 3 | Subordinated debt and 2nd lien notes | Market Approach | Adjusted EBITDA Multiple | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	8.5	11.0
Level 3 | Subordinated debt and 2nd lien notes | Recent Transaction | Transaction Price
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 1,875	$ 670
Measurement input	1.000	0.980
Level 3 | Subordinated debt and 2nd lien notes | Recent Transaction | Transaction Price | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	1.000	0.980
Level 3 | Subordinated debt and 2nd lien notes | Valuation, Unadjusted Prices From Independent Pricing Services And Independent Indicative Broker Quotes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 5,821	$ 5,392
Level 3 | Equity shares
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	193,533	192,563
Level 3 | Equity shares | Yield Analysis | Market Yield
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 16,103	$ 8,788
Level 3 | Equity shares | Yield Analysis | Market Yield | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.105	0.146
Level 3 | Equity shares | Yield Analysis | Market Yield | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.142
Level 3 | Equity shares | Yield Analysis | Market Yield | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	0.130	0.146
Level 3 | Equity shares | Market Approach | Adjusted EBITDA Multiple
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 162,274	$ 162,996
Level 3 | Equity shares | Market Approach | Adjusted EBITDA Multiple | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	5.5	4.8
Level 3 | Equity shares | Market Approach | Adjusted EBITDA Multiple | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	27.5	30.0
Level 3 | Equity shares | Market Approach | Adjusted EBITDA Multiple | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	11.1	11.2
Level 3 | Equity shares | Market Approach | Revenue Multiple
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 1,353	$ 1,390
Level 3 | Equity shares | Market Approach | Revenue Multiple | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	6.0	6.5
Level 3 | Equity shares | Market Approach | Revenue Multiple | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	9.3	9.5
Level 3 | Equity shares | Market Approach | Revenue Multiple | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	6.5	6.8
Level 3 | Equity shares | Discounted Cash Flow Analysis | Discount Rate
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 8,027	$ 6,080
Measurement input	0.145	0.142
Level 3 | Equity shares | Discounted Cash Flow Analysis | Discount Rate | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	0.145	0.142
Level 3 | Equity shares | Net Asset Approach | Liabilities
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 2,357	$ 2,131
Measurement input	64,894,600	55,281,800
Level 3 | Equity shares | Net Asset Approach | Liabilities | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	64,894,600	55,281,800
Level 3 | Equity shares | Recent Transaction | Transaction Price
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 749	$ 5,203
Level 3 | Equity shares | Recent Transaction | Transaction Price | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	0.00	1.00
Level 3 | Equity shares | Recent Transaction | Transaction Price | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	1,037.5	10.00
Level 3 | Equity shares | Recent Transaction | Transaction Price | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	438.67	9.64
Level 3 | Equity shares | Valuation, Unadjusted Prices From Independent Pricing Services And Independent Indicative Broker Quotes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 2,670	$ 5,975
Level 3 | Equity warrants
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	1,102	1,043
Level 3 | Equity warrants | Market Approach
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 1,102	$ 1,043
Level 3 | Equity warrants | Market Approach | Adjusted EBITDA Multiple | Minimum
Schedule of Equity Method Investments [Line Items]
Measurement input	6.3	6.3
Level 3 | Equity warrants | Market Approach | Adjusted EBITDA Multiple | Maximum
Schedule of Equity Method Investments [Line Items]
Measurement input	12.5	12.5
Level 3 | Equity warrants | Market Approach | Adjusted EBITDA Multiple | Weighted Average
Schedule of Equity Method Investments [Line Items]
Measurement input	7.7	7.5
Level 3 | Structured products
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 15,801	$ 14,915
Level 3 | Structured products | Valuation, Unadjusted Prices From Independent Pricing Services And Independent Indicative Broker Quotes
Schedule of Equity Method Investments [Line Items]
Total investments at fair value	$ 15,801	$ 14,915