Investments - Cost and Fair Value by Type and by Industry (Details) - USD ($) $ in Thousands	Jun. 30, 2024		Dec. 31, 2023
Schedule of Investments [Line Items]
Cost	$ 960,192	[1]	$ 569,247	[2]
Fair Value	973,553	[1]	580,466	[2]
Software
Schedule of Investments [Line Items]
Cost	404,414		197,095
Fair Value	408,582		201,391
Business Services
Schedule of Investments [Line Items]
Cost	292,528		209,656
Fair Value	296,684		212,035
Healthcare
Schedule of Investments [Line Items]
Cost	88,505		48,501
Fair Value	89,164		48,824
Consumer Services
Schedule of Investments [Line Items]
Cost	59,012		29,992
Fair Value	59,408		30,171
Education
Schedule of Investments [Line Items]
Cost	43,057		31,499
Fair Value	43,844		32,917
Financial Services
Schedule of Investments [Line Items]
Cost	35,933		23,573
Fair Value	36,042		23,604
Food & Beverage
Schedule of Investments [Line Items]
Cost	29,817		22,885
Fair Value	32,827		25,436
Distribution & Logistics
Schedule of Investments [Line Items]
Cost	5,491		5,499
Fair Value	5,561		5,575
Packaging
Schedule of Investments [Line Items]
Cost	1,435		547
Fair Value	1,441		513
First lien
Schedule of Investments [Line Items]
Cost	895,677		532,881
Fair Value	905,029		541,000
Second lien
Schedule of Investments [Line Items]
Cost	34,595		14,115
Fair Value	36,795		16,288
Subordinated
Schedule of Investments [Line Items]
Cost	28,511		20,895
Fair Value	30,362		21,812
Equity and other
Schedule of Investments [Line Items]
Cost	1,409		1,356
Fair Value	$ 1,367		$ 1,366

[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.