Annual report [Section 13 and 15(d), not S-K Item 405]

FINANCIAL HIGHLIGHTS (Details)

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FINANCIAL HIGHLIGHTS (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investment Company, Financial Highlights [Roll Forward]                    
NAV as of the beginning of the period (in dollars per share) $ 29.20 $ 26.86 $ 25.29 $ 22.35 $ 23.91 $ 24.09 $ 23.53 $ 22.10 $ 21.24 $ 20.85
Net investment income (in dollars per share) 4.09 4.14 3.29 2.65 2.10 2.50 2.60 2.39 2.23 2.18
Net realized gain (loss) (in dollars per share) 0.53 (1.47) (0.07) 0.66 (1.77) (0.33) (0.03) 0.19 0.56 (0.43)
Net unrealized appreciation (depreciation) (in dollars per share) 1.59 2.84 0.33 1.97 (0.09) (0.09) 0.32 0.86 (0.14) 0.20
Income tax benefit (provision) (in dollars per share) (0.36) (0.28) (0.31) (0.48) 0.21 (0.02) (0.09) (0.43) 0.02 0.18
Net increase in net assets resulting from operations (in dollars per share) 5.85 5.23 3.24 4.80 0.45 2.06 2.80 3.01 2.67 2.13
Dividends paid from net investment income (in dollars per share) (4.11) (3.70) (2.95) (2.58) (2.46) (2.91) (2.69) (2.47) (1.99) (2.49)
Distributions from capital gains (in dollars per share)           0 (0.16) (0.32) (0.74) (0.16)
Dividends paid (in dollars per share) (4.11) (3.70) (2.95) (2.58) (2.46) (2.91) (2.85) (2.79) (2.73) (2.65)
Impact of the net change in monthly dividends declared prior to the end of the period and paid in the subsequent period (in dollars per share) (0.01) (0.01) (0.01) (0.01) 0 (0.01) (0.01) (0.01) (0.01) (0.01)
Accretive effect of stock offerings (issuing shares above NAV per share) (in dollars per share) 0.51 0.67 1.17 0.58 0.41 0.55 0.47 1.07 0.76 0.74
Accretive effect of DRIP issuance (issuing shares above NAV per share) (in dollars per share) 0.15 0.10 0.09 0.09 0.08 0.12 0.09 0.06 0.08 0.12
Other (in dollars per share) 0.06 0.05 0.03 0.06 (0.04) 0.01 0.06 0.09 0.09 0.06
NAV as of the end of the period (in dollars per share) 31.65 29.20 26.86 25.29 22.35 23.91 24.09 23.53 22.10 21.24
Market value as of the end of the period (in dollars per share) $ 58.58 $ 43.23 $ 36.95 $ 44.86 $ 32.26 $ 43.11 $ 33.81 $ 39.73 $ 36.77 $ 29.08
Shares outstanding as of the end of the period (in shares) 88,400,391 84,833,002 78,506,816 70,737,021 67,762,032 64,252,937 61,264,861 58,660,680 54,354,857 50,413,744
NAV at end of period $ 2,797,838 $ 2,477,399 $ 2,108,586 $ 1,788,846 $ 1,514,767 $ 1,536,390 $ 1,476,049 $ 1,380,368 $ 1,201,481 $ 1,070,894
Average NAV 2,612,483 2,276,932 1,923,134 1,626,585 1,436,291 1,517,615 1,441,163 1,287,639 1,118,567 1,053,313
Average outstanding debt $ 2,128,092 $ 1,951,923 $ 1,882,462 $ 1,417,831 $ 1,152,108 $ 1,055,800 $ 947,694 $ 843,993 $ 801,048 $ 759,396
Investment Company, Ratio To Average Net Assets [Abstract]                    
Ratio of total expenses, including income tax expense, to average NAV 8.29% 8.08% 8.05% 8.56% 4.95% 5.75% 5.75% 7.37% 5.48% 4.63%
Ratio of operating expenses to average NAV 7.12% 7.09% 6.84% 6.54% 5.89% 5.67% 5.32% 5.47% 5.59% 5.45%
Ratio of operating expenses, excluding interest expense, to average NAV 2.39% 2.58% 2.77% 2.92% 2.44% 2.36% 2.30% 2.63% 2.58% 2.41%
Ratio of net investment income to average NAV (as a percent) 13.59% 14.89% 12.76% 11.23% 9.60% 10.37% 10.87% 10.51% 10.35% 10.15%
Portfolio turnover ratio (as a percent) 22.33% 19.24% 16.79% 29.81% 18.00% 18.86% 29.13% 38.18% 24.63% 25.37%
Total investment return 47.24% 28.23% (11.18%) 48.24% (19.11%) 36.86% (8.25%) 16.02% 37.36% 8.49%
Total return based on change in NAV (as a percent) 20.51% 20.32% 13.51% 21.84% 1.91% 8.78% 12.19% 14.20% 12.97% 11.11%
Related Party | External Investment Manager                    
Investment Company, Ratio To Average Net Assets [Abstract]                    
Total expenses $ 23,100 $ 22,100 $ 13,000 $ 10,300 $ 7,400 $ 6,700 $ 6,800 $ 6,400 $ 5,100 $ 4,300