Investments - Investment Cost and Fair Value by Type and by Industry (Details) - USD ($)
$ in Thousands
Sep. 30, 2022
Dec. 31, 2021
Schedule of Investments [Line Items]    
Cost $ 3,258,903 [1] $ 3,126,492 [2]
Fair Value 3,234,433 [1] 3,174,364 [2]
Software    
Schedule of Investments [Line Items]    
Cost 898,673 782,714
Fair Value 885,447 781,304
Business Services    
Schedule of Investments [Line Items]    
Cost 566,853 578,635
Fair Value 494,874 514,013
Healthcare Services    
Schedule of Investments [Line Items]    
Cost 520,449 510,832
Fair Value 482,067 509,941
Investment Funds (includes investments in joint ventures)    
Schedule of Investments [Line Items]    
Cost 252,400 252,400
Fair Value 252,400 252,400
Education    
Schedule of Investments [Line Items]    
Cost 198,050 200,895
Fair Value 247,882 250,351
Consumer Services    
Schedule of Investments [Line Items]    
Cost 178,805 111,464
Fair Value 178,097 111,140
Net Lease    
Schedule of Investments [Line Items]    
Cost 103,387 150,603
Fair Value 134,911 229,253
Specialty Chemicals & Materials    
Schedule of Investments [Line Items]    
Cost 58,430 60,295
Fair Value 97,959 60,367
Distribution & Logistics    
Schedule of Investments [Line Items]    
Cost 106,081 106,211
Fair Value 96,759 104,112
Financial Services    
Schedule of Investments [Line Items]    
Cost 76,961 55,424
Fair Value 76,911 55,745
Healthcare Information Technology    
Schedule of Investments [Line Items]    
Cost 69,009 52,804
Fair Value 68,858 52,946
Information Technology    
Schedule of Investments [Line Items]    
Cost 58,621 58,570
Fair Value 58,553 58,553
Energy    
Schedule of Investments [Line Items]    
Cost 54,262 47,702
Fair Value 50,319 38,759
Packaging    
Schedule of Investments [Line Items]    
Cost 42,121 34,763
Fair Value 40,689 33,723
Consumer Products    
Schedule of Investments [Line Items]    
Cost 20,750 10,218
Fair Value 20,737 10,206
Federal Services    
Schedule of Investments [Line Items]    
Cost 12,700 12,797
Fair Value 12,372 12,790
Insurance Services    
Schedule of Investments [Line Items]    
Cost 15,634 76,307
Fair Value 11,990 75,094
Business Products    
Schedule of Investments [Line Items]    
Cost 10,767 10,764
Fair Value 8,787 10,586
Industrial Services    
Schedule of Investments [Line Items]    
Cost 7,863 7,368
Fair Value 7,823 7,362
Information Services    
Schedule of Investments [Line Items]    
Cost 7,087 5,726
Fair Value 6,998 5,719
First lien    
Schedule of Investments [Line Items]    
Cost 1,811,879 1,682,541
Fair Value 1,770,125 1,657,815
Second lien    
Schedule of Investments [Line Items]    
Cost 642,943 645,370
Fair Value 573,535 627,356
Subordinated    
Schedule of Investments [Line Items]    
Cost 80,484 54,996
Fair Value 73,763 50,742
Equity and other    
Schedule of Investments [Line Items]    
Cost 723,597 743,585
Fair Value $ 817,010 $ 838,451
[1] New Mountain Finance Corporation (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act
[2] New Mountain Finance Corporation (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act.