v3.26.1
Net Assets - Schedule of Net Assets (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Jun. 30, 2026
Jun. 30, 2025
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period $ 1,049,970 $ 1,188,256 $ 1,348,178 $ 1,359,291 $ 1,188,256 $ 1,359,291
Repurchases of common stock under the Repurchase Program (in shares) (1,122,761)   (925,216)   (8,185,998) (925,216)
Repurchases of common stock under the Repurchase Program $ (8,966) (56,597) $ (9,642)   $ (65,563) $ (9,642)
Conversion of 2022 Convertible Notes (in shares) 0   514   0 514
Offering costs       (28)    
Offering costs     $ 14      
Distributions declared $ (23,702) (31,240) (34,589) (34,602)    
Contributions related to non-controlling interest in NMNLC   300        
Net increase (decrease) in net assets resulting from operations 17,513 (50,749) 7,875 23,517 $ (33,236) $ 31,392
Net assets at the end of the period 1,034,815 1,049,970 1,311,836 1,348,178 1,034,815 1,311,836
Total Net Assets of NMFC            
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period 1,043,502 1,182,177 1,342,212 1,353,339 1,182,177 1,353,339
Repurchases of common stock under the Repurchase Program (8,966) (56,597) (9,642)   (65,563) (9,642)
Offering costs 0   14 (28) 0 (14)
Offering costs     14      
Distributions declared (23,612) (31,150) (34,499) (34,512) (54,762) (69,011)
Net increase (decrease) in net assets resulting from operations 17,257 (50,928) 7,774 23,413 (33,671) 31,187
Net assets at the end of the period $ 1,028,181 $ 1,043,502 $ 1,305,859 $ 1,342,212 $ 1,028,181 $ 1,305,859
Common Stock            
Investment Company, Net Assets [Roll Forward]            
Beginning of the period (in shares) 95,575,151 102,638,388 107,851,415 107,851,415 102,638,388 107,851,415
New Mountain Finance Corporation net assets at the beginning of the period $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079 $ 1,079
Repurchases of common stock under the Repurchase Program (in shares) (1,122,761) (7,063,237) (925,216)      
Conversion of 2022 Convertible Notes (in shares)     514      
End of the period (in shares) 94,452,390 95,575,151 106,926,713 107,851,415 94,452,390 106,926,713
Net assets at the end of the period $ 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 (108,549) (51,952) 0 0 (51,952) 0
Repurchases of common stock under the Repurchase Program (8,966) (56,597) (9,642)      
Net assets at the end of the period (117,515) (108,549) (9,642) 0 (117,515) (9,642)
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 1,365,824 1,365,852 1,354,726 1,365,852
Offering costs       (28)    
Offering costs     14      
Net assets at the end of the period 1,354,726 1,354,726 1,365,838 1,365,824 1,354,726 1,365,838
Accumulated Net Investment Income            
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period 191,950 192,583 181,278 181,266 192,583 181,266
Distributions declared (23,612) (31,150) (34,499) (34,512)    
Net increase (decrease) in net assets resulting from operations 24,502 30,517 34,468 34,524    
Net assets at the end of the period 192,840 191,950 181,247 181,278 192,840 181,247
Accumulated Net Realized (Losses) Gains            
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period (130,131) (98,121) (103,454) (141,279) (98,121) (141,279)
Net increase (decrease) in net assets resulting from operations (44,984) (32,010) 13,389 37,825    
Net assets at the end of the period (175,115) (130,131) (90,065) (103,454) (175,115) (90,065)
Net Unrealized Appreciation (Depreciation)            
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period (265,573) (216,138) (102,515) (53,579) (216,138) (53,579)
Net increase (decrease) in net assets resulting from operations 37,739 (49,435) (40,083) (48,936)    
Net assets at the end of the period (227,834) (265,573) (142,598) (102,515) (227,834) (142,598)
Non-Controlling Interest in NMNLC            
Investment Company, Net Assets [Roll Forward]            
New Mountain Finance Corporation net assets at the beginning of the period 6,468 6,079 5,966 5,952 6,079 5,952
Distributions declared (90) (90) (90) (90)    
Contributions related to non-controlling interest in NMNLC   300        
Net increase (decrease) in net assets resulting from operations 256 179 101 104 435 205
Net assets at the end of the period $ 6,634 $ 6,468 $ 5,977 $ 5,966 $ 6,634 $ 5,977