Fair Value - Unobservable Inputs (Details) $ in Thousands	Jun. 30, 2024 USD ($)		Dec. 31, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 973,553	[1]	$ 580,466	[2]
First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	905,029		541,000
Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	36,795		16,288
Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	30,362		21,812
Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,367		1,366
Level III
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	858,720		475,705
Level III | First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	826,750		459,051
Level III | First lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 736,480		$ 393,056
Level III | First lien | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	8.5		5.0
Level III | First lien | Market & income approach | Low | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	4.1		3.8
Level III | First lien | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.078		0.072
Level III | First lien | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	24.5		27.0
Level III | First lien | Market & income approach | High | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	13.0		11.8
Level III | First lien | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.188		0.186
Level III | First lien | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	15.7		17.8
Level III | First lien | Market & income approach | Weighted Average | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	8.1		9.1
Level III | First lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.103		0.101
Level III | First lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 90,270		$ 65,995
Level III | Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	18,016		8,678
Level III | Second lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 8,808		$ 8,678
Level III | Second lien | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	14.0		16.0
Level III | Second lien | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.110		0.092
Level III | Second lien | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	18.0		18.0
Level III | Second lien | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.111		0.106
Level III | Second lien | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	16.3		16.5
Level III | Second lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.111		0.103
Level III | Second lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 9,208
Level III | Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	12,587		$ 6,610
Level III | Subordinated | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 12,587		$ 6,610
Level III | Subordinated | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	14.0		20.0
Level III | Subordinated | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.132		0.129
Level III | Subordinated | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	24.5		22.0
Level III | Subordinated | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.158		0.158
Level III | Subordinated | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	18.0		21.0
Level III | Subordinated | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.149		0.142
Level III | Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,367		$ 1,366
Level III | Equity and other | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,367		$ 1,366
Level III | Equity and other | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input			24.0
Level III | Equity and other | Market & income approach | Low | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	4.1		3.8
Level III | Equity and other | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.158		0.148
Level III | Equity and other | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input			24.0
Level III | Equity and other | Market & income approach | High | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	6.0		4.2
Level III | Equity and other | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.158		0.186
Level III | Equity and other | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input			24.0
Level III | Equity and other | Market & income approach | Weighted Average | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	5.1		4.0
Level III | Equity and other | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.158		0.161

[1] New Mountain Guardian IV BDC, L.L.C. (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 Guardian IV BDC, L.L.C. (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.