v3.26.3
Net Assets - Schedule of Net Assets (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Jun. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Sep. 30, 2026
Sep. 30, 2025
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period $ 1,034,815 $ 1,049,970 $ 1,188,256 $ 1,311,836 $ 1,348,178 $ 1,359,291 $ 1,188,256 $ 1,359,291
Repurchases of common stock under the Repurchase Program (in shares) 0     (2,737,377)     (8,185,998) (3,662,593)
Repurchases of common stock under the Repurchase Program   (8,966) (56,597) $ (27,611) (9,642)   $ (65,563) $ (37,253)
Conversion of 2022 Convertible Notes (in shares) 0     0     0 514
Conversion of 2022 Convertible Notes to Shares       $ 7        
Offering costs           (28)    
Offering costs       4 14      
Distributions declared $ (24,350) (23,702) (31,240) (33,912) (34,589) (34,602)    
Contributions related to non-controlling interest in NMNLC     300          
Net increase (decrease) in net assets resulting from operations 3,743 17,513 (50,749) 12,342 7,875 23,517 $ (29,493) $ 43,734
New Mountain Finance Corporation net assets at the end of the period 1,014,208 1,034,815 1,049,970 1,262,666 1,311,836 1,348,178 1,014,208 1,262,666
Total Net Assets of NMFC                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period 1,028,181 1,043,502 1,182,177 1,305,859 1,342,212 1,353,339 1,182,177 1,353,339
Repurchases of common stock under the Repurchase Program 0 (8,966) (56,597) (27,611) (9,642)   (65,563) (37,253)
Conversion of 2022 Convertible Notes to Shares 0     7     0 7
Offering costs 0     4   (28) 0 (10)
Offering costs       4 14      
Distributions declared (23,613) (23,612) (31,150) (33,822) (34,499) (34,512) (78,375) (102,833)
Net increase (decrease) in net assets resulting from operations 3,479 17,257 (50,928) 12,189 7,774 23,413 (30,192) 43,376
New Mountain Finance Corporation net assets at the end of the period $ 1,008,047 $ 1,028,181 $ 1,043,502 $ 1,256,626 $ 1,305,859 $ 1,342,212 $ 1,008,047 $ 1,256,626
Common Stock                
Investment Company, Net Assets [Roll Forward]                
Beginning of the period (in shares) 94,452,390 95,575,151 102,638,388 106,926,713 107,851,415 107,851,415 102,638,388 107,851,415
New Mountain Finance Corporation net assets at the beginning of the period $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079
Repurchases of common stock under the Repurchase Program (in shares)   (1,122,761) (7,063,237) (2,737,377) (925,216)      
Conversion of 2022 Convertible Notes (in shares)         514      
End of the period (in shares) 94,452,390 94,452,390 95,575,151 104,189,336 106,926,713 107,851,415 94,452,390 104,189,336
New Mountain Finance Corporation net assets at the end of the period $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079
Treasury Stock at Cost                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period (117,515) (108,549) (51,952) (9,642) 0 0 (51,952) 0
Repurchases of common stock under the Repurchase Program   (8,966) (56,597) (27,611) (9,642)      
New Mountain Finance Corporation net assets at the end of the period (117,515) (117,515) (108,549) (37,253) (9,642) 0 (117,515) (37,253)
Paid in Capital in Excess of Par                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period 1,354,726 1,354,726 1,354,726 1,365,838 1,365,824 1,365,852 1,354,726 1,365,852
Conversion of 2022 Convertible Notes to Shares       7        
Offering costs           (28)    
Offering costs       4 14      
New Mountain Finance Corporation net assets at the end of the period 1,354,726 1,354,726 1,354,726 1,365,849 1,365,838 1,365,824 1,354,726 1,365,849
Accumulated Net Investment Income                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period 192,840 191,950 192,583 181,247 181,278 181,266 192,583 181,266
Distributions declared (23,613) (23,612) (31,150) (33,822) (34,499) (34,512)    
Net increase (decrease) in net assets resulting from operations 23,580 24,502 30,517 33,925 34,468 34,524    
New Mountain Finance Corporation net assets at the end of the period 192,807 192,840 191,950 181,350 181,247 181,278 192,807 181,350
Accumulated Net Realized (Losses) Gains                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period (175,115) (130,131) (98,121) (90,065) (103,454) (141,279) (98,121) (141,279)
Net increase (decrease) in net assets resulting from operations (532) (44,984) (32,010) 9 13,389 37,825    
New Mountain Finance Corporation net assets at the end of the period (175,647) (175,115) (130,131) (90,056) (90,065) (103,454) (175,647) (90,056)
Net Unrealized Appreciation (Depreciation)                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period (227,834) (265,573) (216,138) (142,598) (102,515) (53,579) (216,138) (53,579)
Net increase (decrease) in net assets resulting from operations (19,569) 37,739 (49,435) (21,745) (40,083) (48,936)    
New Mountain Finance Corporation net assets at the end of the period (247,403) (227,834) (265,573) (164,343) (142,598) (102,515) (247,403) (164,343)
Non-Controlling Interest in NMNLC                
Investment Company, Net Assets [Roll Forward]                
New Mountain Finance Corporation net assets at the beginning of the period 6,634 6,468 6,079 5,977 5,966 5,952 6,079 5,952
Distributions declared (737) (90) (90) (90) (90) (90)    
Contributions related to non-controlling interest in NMNLC     300          
Net increase (decrease) in net assets resulting from operations 264 256 179 153 101 104 699 358
New Mountain Finance Corporation net assets at the end of the period $ 6,161 $ 6,634 $ 6,468 $ 6,040 $ 5,977 $ 5,966 $ 6,161 $ 6,040