Formation and Business Purpose	12 Months Ended
Formation and Business Purpose	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business Purpose	Formation and Business Purpose New Mountain Finance Corporation (“NMFC” or the “Company”) is a Delaware corporation that was originally incorporated on June 29, 2010 and completed its initial public offering ("IPO") on May 19, 2011. NMFC is a closed-end, non-diversified management investment company that has elected to be regulated as a business development company (“BDC”) under the Investment Company Act of 1940, as amended (the “1940 Act”). NMFC has elected to be treated, and intends to comply with the requirements to continue to qualify annually, as a regulated investment company (“RIC”) under Subchapter M of the Internal Revenue Code of 1986, as amended (the “Code”). Since NMFC’s IPO, and through December 31, 2022, NMFC raised approximately $945,617 in net proceeds from additional offerings of its common stock. New Mountain Finance Advisers BDC, L.L.C. (the “Investment Adviser”) is a wholly-owned subsidiary of New Mountain Capital Group, L.P. (together with New Mountain Capital, L.L.C. and its affiliates, "New Mountain Capital") whose ultimate owners include Steven B. Klinsky, other current and former New Mountain Capital professionals and related vehicles and a minority investor. New Mountain Capital is a firm with a track record of investing in the middle market. New Mountain Capital focuses on investing in defensive growth companies across its private equity, credit and net lease investment strategies. The Investment Adviser manages the Company's day-to-day operations and provides it with investment advisory and management services. The Investment Adviser also manages other funds that may have investment mandates that are similar, in whole or in part, to the Company's. New Mountain Finance Administration, L.L.C. (the "Administrator”), a wholly-owned subsidiary of New Mountain Capital, provides the administrative services necessary to conduct the Company's day-to-day operations. The Company has established the following wholly-owned direct and indirect subsidiaries: •New Mountain Finance Holdings, L.L.C. ("NMF Holdings") and New Mountain Finance DB, L.L.C. ("NMFDB"), whose assets are used to secure NMF Holdings’ credit facility and NMFDB’s credit facility, respectively; •New Mountain Finance SBIC, L.P. ("SBIC I") and New Mountain Finance SBIC II, L.P. ("SBIC II"), who have received licenses from the U.S. Small Business Administration ("SBA") to operate as small business investment companies ("SBICs") under Section 301(c) of the Small Business Investment Act of 1958, as amended (the "1958 Act"), and their general partners, New Mountain Finance SBIC G.P., L.L.C. ("SBIC I GP"), and New Mountain Finance SBIC II G.P., L.L.C. ("SBIC II GP"), respectively; •NMF Ancora Holdings, Inc. ("NMF Ancora"), NMF QID NGL Holdings, Inc. ("NMF QID"), NMF YP Holdings, Inc. ("NMF YP"), NMF Permian Holdings, LLC ("NMF Permian"), NMF HB, Inc. ("NMF HB"), NMF TRM, LLC ("NMF TRM"), NMF Pioneer, Inc. ("NMF Pioneer") and NMF OEC, Inc. ("NMF OEC"), which serve as tax blocker corporations by holding equity or equity-like investments in portfolio companies organized as limited liability companies (or other forms of pass-through entities); the Company consolidates its tax blocker corporations for accounting purposes but the tax blocker corporations are not consolidated for U.S. federal income tax purposes and may incur U.S. federal income tax expense as a result of their ownership of the portfolio companies; and •New Mountain Finance Servicing, L.L.C. ("NMF Servicing"), which serves as the administrative agent on certain investment transactions. New Mountain Net Lease Corporation ("NMNLC") is a majority-owned consolidated subsidiary of the Company, which acquires commercial real estate properties that are subject to "triple net" leases and has elected to be treated, and intends to comply with the requirements to continue to qualify annually, as a real estate investment trust, or REIT, within the meaning of Section 856(a) of the Code. The Company’s investment objective is to generate current income and capital appreciation through the sourcing and origination of debt securities at all levels of the capital structure, including first and second lien debt, notes, bonds and mezzanine securities. The first lien debt may include traditional first lien senior secured loans or unitranche loans. Unitranche loans will expose the Company to the risks associated with second lien and subordinated loans to the extent the Company invests in the “last out” tranche. The Company invests a significant portion of its portfolio in unitranche loans, which are loans that combine both senior and subordinated debt, generally in a first-lien position. Because unitranche loans combine characteristics of senior and subordinated debt, they have risks similar to the risks associated with secured debt and subordinated debt according to the combination of loan characteristics of the unitranche loan. Certain unitranche loan investments may include “last-out” positions, which generally heighten the risk of loss. Unitranche loans generally allow the borrower to make a large lump sum payment of principal at the end of the loan term and there is a heightened risk of loss if the borrower is unable to pay the lump sum or refinance the amount owed at maturity. In some cases, the Company’s investments may also include equity interests. The Company’s primary focus is in the debt of defensive growth companies, which are defined as generally exhibiting the following characteristics: (i) sustainable secular growth drivers, (ii) high barriers to competitive entry, (iii) high free cash flow after capital expenditure and working capital needs, (iv) high returns on assets and (v) niche market dominance. Similar to the Company, SBIC I's and SBIC II's investment objectives are to generate current income and capital appreciation under the investment criteria used by the Company. However, SBIC I and SBIC II investments must be in SBA eligible small businesses. The Company’s portfolio may be concentrated in a limited number of industries. As of December 31, 2022, the Company’s top five industry concentrations were software, business services, healthcare, investment funds (which includes the Company's investments in its joint ventures) and education.