Net Assets - Roll forward of Net Assets (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | ||||||
|---|---|---|---|---|---|---|---|---|
Sep. 30, 2023 |
Jun. 30, 2023 |
Mar. 31, 2023 |
Sep. 30, 2022 |
Jun. 30, 2022 |
Mar. 31, 2022 |
Sep. 30, 2023 |
Sep. 30, 2022 |
|
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period (in shares) | 100,937,026 | 100,937,026 | ||||||
| Beginning of the period | $ 1,337,962 | $ 1,338,494 | $ 1,326,191 | $ 1,365,201 | $ 1,367,840 | $ 1,342,612 | $ 1,326,191 | $ 1,342,612 |
| Issuances of common stock | 6,625 | 2,955 | 16,577 | 21,572 | ||||
| Offering costs | (66) | (91) | (56) | (34) | (74) | (52) | ||
| Distributions declared | (36,526) | (35,519) | (32,453) | (30,538) | (34,405) | (33,339) | ||
| Contributions related to non-controlling interest in NMNLC | 123 | |||||||
| Net increase (decrease) in net assets resulting from operations | $ 28,699 | 35,078 | 44,812 | 7,514 | 15,140 | 37,047 | $ 108,589 | 59,701 |
| End of the period (in shares) | 101,439,649 | 101,439,649 | ||||||
| Ending of the period | $ 1,336,694 | 1,337,962 | 1,338,494 | 1,345,098 | 1,365,201 | 1,367,840 | $ 1,336,694 | 1,345,098 |
| Total Net Assets of NMFC | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | 1,326,101 | 1,326,690 | 1,314,473 | 1,351,610 | 1,349,368 | 1,321,245 | 1,314,473 | 1,321,245 |
| Issuances of common stock | 6,625 | 2,955 | 16,577 | 21,572 | ||||
| Offering costs | (66) | (91) | (56) | (34) | (74) | (52) | (213) | (160) |
| Distributions declared | (36,338) | (35,328) | (32,300) | (30,281) | (30,215) | (29,589) | (103,966) | (90,085) |
| Net increase (decrease) in net assets resulting from operations | 28,696 | 34,830 | 44,573 | 7,705 | 15,954 | 36,192 | 108,099 | 59,851 |
| Ending of the period | $ 1,325,018 | $ 1,326,101 | $ 1,326,690 | $ 1,331,955 | $ 1,351,610 | $ 1,349,368 | $ 1,325,018 | $ 1,331,955 |
| Common Stock | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period (in shares) | 100,937,026 | 100,937,026 | 100,937,026 | 100,716,928 | 99,498,562 | 97,907,441 | 100,937,026 | 97,907,441 |
| Beginning of the period | $ 1,009 | $ 1,009 | $ 1,009 | $ 1,007 | $ 995 | $ 979 | $ 1,009 | $ 979 |
| Issuances of common stock (in shares) | 502,623 | 220,098 | 1,218,366 | 1,591,121 | ||||
| Issuances of common stock | $ 5 | $ 2 | $ 12 | $ 16 | ||||
| End of the period (in shares) | 101,439,649 | 100,937,026 | 100,937,026 | 100,937,026 | 100,716,928 | 99,498,562 | 101,439,649 | 100,937,026 |
| Ending of the period | $ 1,014 | $ 1,009 | $ 1,009 | $ 1,009 | $ 1,007 | $ 995 | $ 1,014 | $ 1,009 |
| Paid in Capital in Excess of Par | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | 1,305,798 | 1,305,889 | 1,305,945 | 1,310,791 | 1,294,300 | 1,272,796 | 1,305,945 | 1,272,796 |
| Issuances of common stock | 6,620 | 2,953 | 16,565 | 21,556 | ||||
| Offering costs | (66) | (91) | (56) | (34) | (74) | (52) | ||
| Ending of the period | 1,312,352 | 1,305,798 | 1,305,889 | 1,313,710 | 1,310,791 | 1,294,300 | 1,312,352 | 1,313,710 |
| Accumulated Net Investment Income | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | 157,756 | 153,406 | 147,593 | 119,495 | 118,314 | 118,330 | 147,593 | 118,330 |
| Distributions declared | (36,338) | (35,328) | (32,300) | (30,281) | (30,215) | (29,589) | ||
| Net increase (decrease) in net assets resulting from operations | 40,445 | 39,678 | 38,113 | 32,527 | 31,396 | 29,573 | ||
| Ending of the period | 161,863 | 157,756 | 153,406 | 121,741 | 119,495 | 118,314 | 161,863 | 121,741 |
| Accumulated Net Realized (Losses) Gains | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | (64,828) | (67,395) | (68,072) | (59,654) | (74,503) | (92,099) | (68,072) | (92,099) |
| Net increase (decrease) in net assets resulting from operations | 4,673 | 2,567 | 677 | (390) | 14,849 | 17,596 | ||
| Ending of the period | (60,155) | (64,828) | (67,395) | (60,044) | (59,654) | (74,503) | (60,155) | (60,044) |
| Net Unrealized Appreciation (Depreciation) | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | (73,634) | (66,219) | (72,002) | (20,029) | 10,262 | 21,239 | (72,002) | 21,239 |
| Net increase (decrease) in net assets resulting from operations | (16,422) | (7,415) | 5,783 | (24,432) | (30,291) | (10,977) | ||
| Ending of the period | (90,056) | (73,634) | (66,219) | (44,461) | (20,029) | 10,262 | (90,056) | (44,461) |
| Non-Controlling Interest in NMNLC | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of the period | 11,861 | 11,804 | 11,718 | 13,591 | 18,472 | 21,367 | 11,718 | 21,367 |
| Distributions declared | (188) | (191) | (153) | (257) | (4,190) | (3,750) | ||
| Contributions related to non-controlling interest in NMNLC | 123 | |||||||
| Net increase (decrease) in net assets resulting from operations | 3 | 248 | 239 | (191) | (814) | 855 | 490 | (150) |
| Ending of the period | $ 11,676 | $ 11,861 | $ 11,804 | $ 13,143 | $ 13,591 | $ 18,472 | $ 11,676 | $ 13,143 |