v3.26.1
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2026
Dec. 31, 2025
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Dec. 31, 2024
Sep. 30, 2024
Jun. 30, 2024
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2022
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Quarterly Financial Information Disclosure [Abstract]                                    
Net investment income $ 34,609 $ 36,985 $ 31,984 $ 31,889 $ 27,842 $ 30,316 $ 31,165 $ 28,859 $ 135,467 $ 118,182 $ 110,005              
Net realized gain (loss) on investments, net of tax (5,485) 225 (4,926) 15,704 (27,267) (12,805) (10,289) 711 5,518 (49,650) (39,895)              
Net change in unrealized (depreciation) appreciation on investments, net of tax (1,645) (2,158) (1,437) (20,592) 16,997 (847) 1,808 (15,535) (25,832) 2,423 13,640              
Realized loss on extinguishment of debt 0 (2,156) 0 0 0 (387) 0 0 (2,156) (387) (361)              
Realized loss on disposal of fixed assets 0 0 (2) 0 (11) (9) 0 0 (2) (20) 0              
Net increase in net assets from operations $ 27,479 $ 32,896 $ 25,619 $ 27,001 $ 17,561 $ 16,268 $ 22,684 $ 14,035 $ 112,995 $ 70,548 $ 83,389              
Pre-tax net investment income per share - basic (in usd per share) $ 0.59 $ 0.60 $ 0.61 $ 0.61 $ 0.56 $ 0.64 $ 0.64 $ 0.69 $ 2.39 $ 2.50 $ 2.72              
Net investment income per share – basic (in usd per share) 0.57 0.64 0.57 0.59 0.54 0.63 0.66 0.63 2.37 2.46 2.70 $ 2.29 $ 1.87 $ 1.66 $ 1.57 $ 1.42 $ 1.01 $ 0.50
Net investment income per share – basic (in usd per share)                 1.98 1.47 $ 2.05              
Net increase in net assets from operations per share - basic (in usd per share) 0.46 0.57 0.46 0.50 0.34 0.34 0.48 0.31 1.98 1.47                
Net increase in net assets from operations per share - diluted (in usd per share) $ 0.44 $ 0.54 $ 0.44 $ 0.48 $ 0.35 $ 0.34 $ 0.48 $ 0.31 $ 1.90 $ 1.47