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Summary of Significant Accounting Policies - Schedule of Properties (Details) - New Mountain Net Lease Corporation ft² in Thousands, $ in Thousands	Jun. 30, 2025 USD ($) ft²
Property, Plant and Equipment [Line Items]
Fair value	$ 114,240
NM NL Holdings LP / NM GP Holdco LLC
Property, Plant and Equipment [Line Items]
Fair value	$ 105,366
NM YI, LLC
Property, Plant and Equipment [Line Items]
Total square feet | ft²	212
Fair value	$ 8,874

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Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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