Annual report pursuant to Section 13 and 15(d)

Schedule 12-14s - CONSOLIDATED SCHEDULE OF INVESTMENTS IN AND ADVANCES TO AFFILIATES (Parenthetical) (Details)

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Schedule 12-14s - CONSOLIDATED SCHEDULE OF INVESTMENTS IN AND ADVANCES TO AFFILIATES (Parenthetical) (Details) - USD ($)
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Investments in and Advances to Affiliates [Line Items]      
Investment cost $ 3,247,033,000 $ 3,005,696,000  
Investments, at fair value: $ 3,248,046,000 $ 2,963,955,000  
Control investments      
Investments in and Advances to Affiliates [Line Items]      
Investment owned, percent of net assets 6.38% 5.46%  
Investment cost $ 103,182,000 $ 87,271,000  
Investments, at fair value: $ 115,004,000 $ 76,458,000 $ 73,504,000
Control investments | Majority Owned Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment owned, percent of net assets 5.74% 4.88%  
Investment cost $ 80,769,000 $ 64,858,000  
Investments, at fair value: $ 103,487,000 $ 68,416,000 65,235,000
Control investments | Other Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment owned, percent of net assets 0.64% 0.58%  
Investment cost $ 22,413,000 $ 22,413,000  
Investments, at fair value: 11,517,000 8,042,000 8,269,000
Control investments | Coronado Aesthetics, LLC      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 262,000 319,000 565,000
Control investments | Coronado Aesthetics, LLC | Majority Owned Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 260,000 313,000 500,000
Control investments | Coronado Aesthetics, LLC | Majority Owned Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 2,000 6,000 65,000
Control investments | Coronado Aesthetics, LLC | Medical Devices & Equipment | Majority Owned Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment cost 250,000 250,000  
Investments, at fair value: $ 262,000 $ 319,000  
Control investments | Coronado Aesthetics, LLC | Medical Devices & Equipment | Majority Owned Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares 180,000 180,000  
Investment cost $ 0 $ 0  
Investments, at fair value: $ 2,000 $ 6,000  
Control investments | Coronado Aesthetics, LLC | Medical Devices & Equipment | Preferred Series A | Majority Owned Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares 5,000,000 5,000,000  
Investment cost $ 250,000 $ 250,000  
Investments, at fair value: 260,000 313,000  
Control investments | Gibraltar Acquisition LLC      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 62,512,000    
Control investments | Gibraltar Acquisition LLC | Majority Owned Control Investments | Unsecured Debt      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 34,478,000 21,700,000  
Control investments | Gibraltar Acquisition LLC | Majority Owned Control Investments | Member Units      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 28,034,000 15,244,000  
Control investments | Gibraltar Acquisition LLC | Diversified Financial Services | Majority Owned Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment cost 68,484,000    
Investments, at fair value: $ 62,512,000    
Control investments | Gibraltar Acquisition LLC | Diversified Financial Services | Majority Owned Control Investments | Unsecured Debt One      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date 2026-09    
Interest rate and floor, fixed 11.50%    
Principal amount $ 25,000,000    
Investment cost 24,663,000    
Investments, at fair value: $ 24,663,000    
Control investments | Gibraltar Acquisition LLC | Diversified Financial Services | Majority Owned Control Investments | Unsecured Debt Two      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date 2026-09    
Interest rate and floor, fixed 11.95%    
Principal amount $ 10,000,000    
Investment cost 9,815,000    
Investments, at fair value: $ 9,815,000    
Control investments | Gibraltar Acquisition LLC | Diversified Financial Services | Majority Owned Control Investments | Member Units      
Investments in and Advances to Affiliates [Line Items]      
Shares 1    
Investment cost $ 34,006,000    
Investments, at fair value: 28,034,000    
Control investments | Hercules Adviser LLC      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 40,713,000 31,153,000 20,840,000
Control investments | Hercules Adviser LLC | Majority Owned Control Investments | Unsecured Debt      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 12,000,000 12,000,000 8,850,000
Control investments | Hercules Adviser LLC | Majority Owned Control Investments | Member Units      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 28,713,000 19,153,000 11,990,000
Control investments | Hercules Adviser LLC | Diversified Financial Services | Majority Owned Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment cost 12,035,000 12,035,000  
Investments, at fair value: $ 40,713,000 $ 31,153,000  
Control investments | Hercules Adviser LLC | Diversified Financial Services | Majority Owned Control Investments | Unsecured Debt      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date 2025-06 2025-06  
Interest rate and floor, fixed 5.00% 5.00%  
Principal amount $ 12,000,000 $ 12,000,000  
Investment cost 12,000,000 12,000,000  
Investments, at fair value: $ 12,000,000 $ 12,000,000  
Control investments | Hercules Adviser LLC | Diversified Financial Services | Majority Owned Control Investments | Member Units      
Investments in and Advances to Affiliates [Line Items]      
Shares 1 1  
Investment cost $ 35,000 $ 35,000  
Investments, at fair value: 28,713,000 19,153,000  
Control investments | Tectura Corporation      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 11,517,000 8,042,000 8,269,000
Control investments | Tectura Corporation | Other Control Investments | Senior Secured Debt      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 8,250,000 8,042,000 8,269,000
Control investments | Tectura Corporation | Other Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 3,263,000 0 0
Control investments | Tectura Corporation | Other Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value: 4,000 0 0
Control investments | Tectura Corporation | Consumer & Business Services | Other Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment cost 22,413,000 22,413,000  
Investments, at fair value: $ 11,517,000 $ 8,042,000  
Control investments | Tectura Corporation | Consumer & Business Services | Other Control Investments | Senior Secured Debt      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date 2024-07 2024-07  
Interest rate and floor, fixed 8.25%    
Interest rate paid in kind   5.00%  
Principal amount $ 8,250,000 $ 10,680,000  
Investment cost 8,250,000 240,000  
Investments, at fair value: 8,250,000 $ 0  
Control investments | Tectura Corporation | Consumer & Business Services | Other Control Investments | Senior Secured Debt One      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date   2024-07  
Interest rate and floor, fixed   8.25%  
Principal amount   $ 8,250,000  
Investment cost   8,250,000  
Investments, at fair value:   $ 8,042,000  
Control investments | Tectura Corporation | Consumer & Business Services | Other Control Investments | Senior Secured Debt Two      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date   2024-07  
Interest rate paid in kind   5.00%  
Principal amount   $ 13,023,000  
Investment cost   13,023,000  
Investments, at fair value:   $ 0  
Control investments | Tectura Corporation | Consumer & Business Services | Other Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares   414,994,863  
Principal amount 414,994,863    
Investment cost 900,000 $ 900,000  
Investments, at fair value: 4,000 $ 0  
Control investments | Tectura Corporation | Consumer & Business Services | Preferred Series BB | Other Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares   1,000,000  
Principal amount 1,000,000    
Investment cost 0 $ 0  
Investments, at fair value: $ 12,000 0  
Control investments | Tectura Corporation | Consumer & Business Services | Preferred Series C | Other Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares 3,235,298    
Investment cost $ 13,263,000    
Investments, at fair value: $ 3,251,000    
Control investments | Gibraltar Business Capital, LLC      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value:   36,944,000 43,830,000
Control investments | Gibraltar Business Capital, LLC | Majority Owned Control Investments | Unsecured Debt      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value:   21,700,000 23,212,000
Control investments | Gibraltar Business Capital, LLC | Majority Owned Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value:   14,137,000 19,393,000
Control investments | Gibraltar Business Capital, LLC | Majority Owned Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Investments, at fair value:   1,107,000 $ 1,225,000
Control investments | Gibraltar Business Capital, LLC | Diversified Financial Services | Majority Owned Control Investments      
Investments in and Advances to Affiliates [Line Items]      
Investment cost   52,573,000  
Investments, at fair value:   $ 36,944,000  
Control investments | Gibraltar Business Capital, LLC | Diversified Financial Services | Majority Owned Control Investments | Unsecured Debt One      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date   2026-09  
Interest rate and floor, fixed   14.50%  
Principal amount   $ 15,000,000  
Investment cost   14,715,000  
Investments, at fair value:   $ 12,802,000  
Control investments | Gibraltar Business Capital, LLC | Diversified Financial Services | Majority Owned Control Investments | Unsecured Debt Two      
Investments in and Advances to Affiliates [Line Items]      
Maturity Date   2026-09  
Interest rate and floor, fixed   11.50%  
Principal amount   $ 10,000,000  
Investment cost   9,852,000  
Investments, at fair value:   $ 8,898,000  
Control investments | Gibraltar Business Capital, LLC | Diversified Financial Services | Majority Owned Control Investments | Common Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares   830,000  
Investment cost   $ 1,884,000  
Investments, at fair value:   $ 1,107,000  
Control investments | Gibraltar Business Capital, LLC | Diversified Financial Services | Preferred Series A | Majority Owned Control Investments | Preferred Stock      
Investments in and Advances to Affiliates [Line Items]      
Shares   10,602,752  
Investment cost   $ 26,122,000  
Investments, at fair value:   $ 14,137,000