Fair Value - Unobservable Inputs (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 580,466	$ 123,211	[1]
First Lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	541,000	99,866
Second Lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	16,288	11,852
Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	21,812	10,730
Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,366	763
Level III
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	475,705	104,427
Level III | First Lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	459,051	93,348
Level III | First Lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 393,056	$ 82,517
Level III | First Lien | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	5.0	10
Level III | First Lien | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	27.0	38
Level III | First Lien | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	17.8	19.8
Level III | First Lien | Market & income approach | Revenue multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	3.8	8
Level III | First Lien | Market & income approach | Revenue multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	11.8	17
Level III | First Lien | Market & income approach | Revenue multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	9.1	10.4
Level III | First Lien | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.072	0.088
Level III | First Lien | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.186	0.125
Level III | First Lien | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.101	0.105
Level III | First Lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 65,995	$ 10,831
Level III | Second Lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	8,678	7,239
Level III | Second Lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 8,678	$ 7,239
Level III | Second Lien | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	16.0	0.14
Level III | Second Lien | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	18.0	0.18
Level III | Second Lien | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	16.5	0.16
Level III | Second Lien | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.092	0.112
Level III | Second Lien | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.106	0.126
Level III | Second Lien | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.103	0.123
Level III | Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6,610	$ 3,077
Level III | Subordinated | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6,610	$ 3,077
Level III | Subordinated | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	20.0	23.5
Level III | Subordinated | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	22.0	23.5
Level III | Subordinated | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	21.0	23.5
Level III | Subordinated | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.129	0.148
Level III | Subordinated | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.158	0.148
Level III | Subordinated | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.142	0.148
Level III | Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,366	$ 763
Level III | Equity and other | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,366	$ 763
Level III | Equity and other | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	24.0	25
Level III | Equity and other | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	24.0	25
Level III | Equity and other | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	24.0	25
Level III | Equity and other | Market & income approach | Revenue multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	3.8
Level III | Equity and other | Market & income approach | Revenue multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	4.2
Level III | Equity and other | Market & income approach | Revenue multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	4.0
Level III | Equity and other | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.148	0.149
Level III | Equity and other | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.186	0.149
Level III | Equity and other | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input	0.161	0.149

[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.