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Net Assets - Roll forward of Net Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period (in shares) | 100,937,026 |  |
Beginning of the period | $ 1,326,191 | $ 1,342,612 | $ 1,236,889
Issuances of common stock | 21,241 | 41,104 | 14,655
Offering costs | (369) | (222) | (231)
Distributions declared | (151,430) | (131,955) | (117,675)
Contributions related to non-controlling interest in NMNLC |  |  | 1,792
Contributions related to non-controlling interest in NMNLC | 0 | 124 |
Net increase (decrease) in net assets resulting from operations | $ 136,090 | $ 74,528 | 207,182
End of the period (in shares) | 102,558,859 | 100,937,026 |
End of the period | $ 1,331,723 | $ 1,326,191 | 1,342,612
Total Net Assets of NMFC |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 1,314,473 | 1,321,245 | 1,221,875
Issuances of common stock | 21,241 | 41,104 | 14,655
Offering costs | (369) | (222) | (231)
Distributions declared | (150,737) | (122,386) | (116,453)
Net increase (decrease) in net assets resulting from operations | 135,343 | 74,732 | 201,399
End of the period | $ 1,319,951 | $ 1,314,473 | $ 1,321,245
Common Stock |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period (in shares) | 100,937,026 | 97,907,441 | 96,827,342
Beginning of the period | $ 1,009 | $ 979 | $ 968
Issuances of common stock (in shares) | 1,621,833 | 3,029,585 | 1,080,099
Issuances of common stock | $ 17 | $ 30 | $ 11
End of the period (in shares) | 102,558,859 | 100,937,026 | 97,907,441
End of the period | $ 1,026 | $ 1,009 | $ 979
Paid in Capital in Excess of Par |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 1,305,945 | 1,272,796 | 1,269,671
Issuances of common stock | 21,224 | 41,074 | 14,644
Offering costs | (369) | (222) | (231)
Tax reclassifications related to return of capital distributions (See Note 10) |  |  | (11,288)
Tax reclassifications related to return of capital distributions (See Note 10) |  | (7,703) |
Tax reclassifications related to return of capital distributions (See Note 10) | 4,469 | (7,703) | (10,476)
End of the period | 1,331,269 | 1,305,945 | 1,272,796
Accumulated Net Investment Income |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 147,593 | 118,330 | 105,981
Distributions declared | (150,737) | (122,386) | (116,453)
Net increase (decrease) in net assets resulting from operations | 158,918 | 118,497 | 117,514
Tax reclassifications related to return of capital distributions (See Note 10) |  |  | 11,288
Tax reclassifications related to return of capital distributions (See Note 10) |  | 33,152 |
Tax reclassifications related to return of capital distributions (See Note 10) | (5,367) | 33,152 | 10,476
End of the period | 150,407 | 147,593 | 118,330
Accumulated Net Realized (Losses) Gains |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | (68,072) | (92,099) | (88,250)
Net increase (decrease) in net assets resulting from operations | (32,841) | 49,476 | (3,849)
Tax reclassifications related to return of capital distributions (See Note 10) |  | (25,449) |
Tax reclassifications related to return of capital distributions (See Note 10) | 898 | (25,449) | 0
End of the period | (100,015) | (68,072) | (92,099)
Net Unrealized Appreciation (Depreciation) |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | (72,002) | 21,239 | (66,495)
Net increase (decrease) in net assets resulting from operations | 9,266 | (93,241) | 87,734
End of the period | (62,736) | (72,002) | 21,239
Non-Controlling Interest in NMNLC |  |  |
Investment Company, Net Assets [Roll Forward] |  |  |
Beginning of the period | 11,718 | 21,367 | 15,014
Distributions declared | (693) | (9,569) | (1,222)
Contributions related to non-controlling interest in NMNLC |  |  | 1,792
Contributions related to non-controlling interest in NMNLC | 0 | 124 |
Net increase (decrease) in net assets resulting from operations | 747 | (204) | 5,783
End of the period | $ 11,772 | $ 11,718 | $ 21,367

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