Formation and Business Purpose	12 Months Ended
Dec. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business Purpose	Formation and Business Purpose NMF SLF I, Inc. (the "Company") is a Maryland corporation formed on January 23, 2019. The Company 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"). The Company has elected to be treated for U.S. federal income tax purposes, and intends to qualify annually, as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"). New Mountain Finance Advisers, L.L.C. (the "Investment Adviser"), formerly known as New Mountain Finance Advisers BDC, L.L.C., 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 global investment firm with approximately $60 billion of assets under management and 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 Administrator has hired a third-party sub-administrator to assist with the provision of administrative services. The Company conducted a private offering (the "Private Offering") of its common stock to investors in reliance on exemptions from the registration requirements of the Securities Act of 1933, as amended (the "Securities Act"). At the closing of any Private Offering, each investor will make a capital commitment (each, a "Capital Commitment") to purchase common stock pursuant to a subscription agreement entered into with the Company (each, a "Subscription Agreement"). Each investor will be required to make capital contributions to purchase the Company's common stock each time a drawdown notice is issued based on such investor's Capital Commitment. The Company commenced its loan origination and investment activities on the date it issued shares to persons not affiliated with the Investment Adviser, which occurred on February 18, 2020 (the "Initial Closing Date"). The Company may conduct subsequent closings at times during its investment period (the "Investment Period"), which commenced on the Initial Closing Date and initially continued until September 30, 2023, subject to automatic extensions thereafter, each for an additional one year period, unless the holders of a majority of the Company's outstanding common stock elect to forego any such extension upon not less than ninety days prior written notice. Effective September 30, 2025, the Investment Period was automatically extended for an additional one year period to September 30, 2026. Pursuant to the Subscription Agreements entered into with each investor, the Company shall commence the wind up of operations two years following the expiration of the Investment Period, subject to additional extensions, each for an additional one year period, upon approval of the holders of a majority of the Company's then outstanding common stock. On December 9, 2020, the Company established NMF SLF I SPV, L.L.C. ("SLF I SPV") as a wholly-owned direct subsidiary, whose assets are used to secure SLF I SPV's credit facility. On October 6, 2022, the Company established NMF SLF I Opportunistic SPV, L.L.C. ("SLF I Opportunistic SPV") as a wholly-owned direct subsidiary. As of December 31, 2025 and December 31, 2024, there were no assets held by SLF I Opportunistic SPV. The Company is focused on providing direct lending solutions to U.S. upper middle market companies backed by top private equity sponsors. The Company’s investment objective is to generate current income and capital appreciation through the sourcing and origination of senior secured loans and select junior capital positions, to growing businesses in defensive industries that offer attractive risk-adjusted returns. The Company’s differentiated investment approach leverages the deep sector knowledge and operating resources of New Mountain Capital. The Company primarily invests in senior secured debt of U.S. sponsor-backed, middle market companies. The Company defines middle market companies as those businesses with annual earnings before interest, taxes, depreciation, and amortization ("EBITDA") between $10,000 and $200,000. The Company focuses on defensive growth businesses that generally exhibit the following characteristics: (i) acyclicality, (ii) sustainable secular growth drivers, (iii) niche market dominance and high barriers to competitive entry, (iv) recurring revenue and strong free cash flow, (v) flexible cost structures and (vi) seasoned management teams. Senior secured loans may include traditional first lien loans or unitranche loans. 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. Certain unitranche loan investments may include “last-out” positions, which generally heighten the risk of loss. As of December 31, 2025, the Company's top five industry concentrations were business services, software, financial services & technology, healthcare and consumer services.

NMF SLF I, Inc. (the "Company") is a Maryland corporation formed on January 23, 2019. The Company 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"). The Company has elected to be treated for U.S. federal income tax purposes, and intends to qualify annually, as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code").

New Mountain Finance Advisers, L.L.C. (the "Investment Adviser"), formerly known as New Mountain Finance Advisers BDC, L.L.C., 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 global investment firm with approximately $60 billion of assets under management and 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 Administrator has hired a third-party sub-administrator to assist with the provision of administrative services.

The Company conducted a private offering (the "Private Offering") of its common stock to investors in reliance on exemptions from the registration requirements of the Securities Act of 1933, as amended (the "Securities Act"). At the closing of any Private Offering, each investor will make a capital commitment (each, a "Capital Commitment") to purchase common stock pursuant to a subscription agreement entered into with the Company (each, a "Subscription Agreement"). Each investor will be required to make capital contributions to purchase the Company's common stock each time a drawdown notice is issued based on such investor's Capital Commitment. The Company commenced its loan origination and investment activities on the date it issued shares to persons not affiliated with the Investment Adviser, which occurred on February 18, 2020 (the "Initial Closing Date"). The Company may conduct subsequent closings at times during its investment period (the "Investment Period"), which commenced on the Initial Closing Date and initially continued until September 30, 2023, subject to automatic extensions thereafter, each for an additional one year period, unless the holders of a majority of the Company's outstanding common stock elect to forego any such extension upon not less than ninety days prior written notice. Effective September 30, 2025, the Investment Period was automatically extended for an additional one year period to September 30, 2026. Pursuant to the Subscription Agreements entered into with each investor, the Company shall commence the wind up of operations two years following the expiration of the Investment Period, subject to additional extensions, each for an additional one year period, upon approval of the holders of a majority of the Company's then outstanding common stock.

On December 9, 2020, the Company established NMF SLF I SPV, L.L.C. ("SLF I SPV") as a wholly-owned direct subsidiary, whose assets are used to secure SLF I SPV's credit facility. On October 6, 2022, the Company established NMF SLF I Opportunistic SPV, L.L.C. ("SLF I Opportunistic SPV") as a wholly-owned direct subsidiary. As of December 31, 2025 and December 31, 2024, there were no assets held by SLF I Opportunistic SPV.

The Company is focused on providing direct lending solutions to U.S. upper middle market companies backed by top private equity sponsors. The Company’s investment objective is to generate current income and capital appreciation through the sourcing and origination of senior secured loans and select junior capital positions, to growing businesses in defensive industries that offer attractive risk-adjusted returns. The Company’s differentiated investment approach leverages the deep sector knowledge and operating resources of New Mountain Capital.

The Company primarily invests in senior secured debt of U.S. sponsor-backed, middle market companies. The Company defines middle market companies as those businesses with annual earnings before interest, taxes, depreciation, and amortization ("EBITDA") between $10,000 and $200,000. The Company focuses on defensive growth businesses that generally exhibit the following characteristics: (i) acyclicality, (ii) sustainable secular growth drivers, (iii) niche market dominance and high barriers to competitive entry, (iv) recurring revenue and strong free cash flow, (v) flexible cost structures and (vi) seasoned management teams.