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Net Assets - Roll forward of Net Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period (in shares) | 102,558,859 |  |
Beginning of the period | $ 1,331,723 | $ 1,326,191 | $ 1,342,612
Issuances of common stock | 67,690 | 21,241 | 41,104
Other | 244 |  |
Offering costs | (552) | (369) | (222)
Distributions declared | (148,118) | (151,430) | (131,955)
Contributions related to non-controlling interest in NMNLC |  |  | 124
Sale of non-controlling interest in NMNLC | (4,666) |  |
Realized gain in NMNLC | (1,533) |  |
Net increase (decrease) in net assets resulting from operations | $ 114,503 | $ 136,090 | 74,528
End of the period (in shares) | 107,851,415 | 102,558,859 |
End of the period | $ 1,359,291 | $ 1,331,723 | 1,326,191
Total Net Assets of NMFC |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 1,319,951 | 1,314,473 | 1,321,245
Issuances of common stock | 67,690 | 21,241 | 41,104
Offering costs | (552) | (369) | (222)
Distributions declared | (147,193) | (150,737) | (122,386)
Net increase (decrease) in net assets resulting from operations | 113,443 | 135,343 | 74,732
End of the period | $ 1,353,339 | $ 1,319,951 | $ 1,314,473
Common Stock |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period (in shares) | 102,558,859 | 100,937,026 | 97,907,441
Beginning of the period | $ 1,026 | $ 1,009 | $ 979
Issuances of common stock (in shares) | 5,292,556 | 1,621,833 | 3,029,585
Issuances of common stock | $ 53 | $ 17 | $ 30
End of the period (in shares) | 107,851,415 | 102,558,859 | 100,937,026
End of the period | $ 1,079 | $ 1,026 | $ 1,009
Paid in Capital in Excess of Par |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 1,331,269 | 1,305,945 | 1,272,796
Issuances of common stock | 67,637 | 21,224 | 41,074
Offering costs | (552) | (369) | (222)
Tax reclassifications related to return of capital distributions (See Note 10) | 3,297 | 4,469 | (7,703)
Tax reclassifications related to return of capital distributions (See Note 10) | (32,502) |  |
End of the period | 1,365,852 | 1,331,269 | 1,305,945
Accumulated Net Investment Income |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 150,407 | 147,593 | 118,330
Distributions declared | (147,193) | (150,737) | (122,386)
Net increase (decrease) in net assets resulting from operations | 145,550 | 158,918 | 118,497
Tax reclassifications related to return of capital distributions (See Note 10) | (3,297) | (5,367) | 33,152
Tax reclassifications related to return of capital distributions (See Note 10) | 32,502 |  |
End of the period | 181,266 | 150,407 | 147,593
Accumulated Net Realized Gains (Losses) |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | (100,015) | (68,072) | (92,099)
Net increase (decrease) in net assets resulting from operations | (41,264) | (32,841) | 49,476
Tax reclassifications related to return of capital distributions (See Note 10) | 0 | 898 | (25,449)
End of the period | (141,279) | (100,015) | (68,072)
Net Unrealized Appreciation (Depreciation) |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | (62,736) | (72,002) | 21,239
Net increase (decrease) in net assets resulting from operations | 9,157 | 9,266 | (93,241)
End of the period | (53,579) | (62,736) | (72,002)
Non-Controlling Interest in NMNLC |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 11,772 | 11,718 | 21,367
Other | 244 |  |
Distributions declared | (925) | (693) | (9,569)
Contributions related to non-controlling interest in NMNLC |  |  | 124
Sale of non-controlling interest in NMNLC | (4,666) |  |
Realized gain in NMNLC | (1,533) |  |
Net increase (decrease) in net assets resulting from operations | 1,060 | 747 | (204)
End of the period | $ 5,952 | $ 11,772 | $ 11,718

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