CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical) - USD ($) $ in Thousands  | 
Jun. 30, 2023  | 
Mar. 31, 2023  | 
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|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cost | $ 767,985 | $ 720,630 | [1] | |||||||||||||||
| Line of credit at cost | $ 46,200 | $ 35,200 | ||||||||||||||||
| Common stock, par value (in USD per share) | $ 0.001 | $ 0.001 | ||||||||||||||||
| Common stock, shares authorized (in shares) | 100,000,000 | 100,000,000 | ||||||||||||||||
| Common stock, shares, issued (In shares) | 33,591,505 | 33,591,505 | ||||||||||||||||
| Common stock, shares, outstanding (in shares) | 33,591,505 | 33,591,505 | ||||||||||||||||
| Non-Control/Non-Affiliate Investments | ||||||||||||||||||
| Cost | $ 444,780 | [2] | $ 429,305 | [3] | ||||||||||||||
| Affiliate Investments | ||||||||||||||||||
| Cost | 308,555 | [4] | 276,055 | [5] | ||||||||||||||
| Control Investments | ||||||||||||||||||
| Cost | $ 14,650 | [6] | $ 15,270 | [7] | ||||||||||||||
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