Investments - Cost and Fair Value by Type and by Industry (Details) - USD ($) $ in Thousands	Sep. 30, 2024		Dec. 31, 2023
Schedule of Investments [Line Items]
Cost	$ 1,231,736	[1]	$ 569,247	[2]
Fair Value	1,244,475	[1]	580,466	[2]
Software
Schedule of Investments [Line Items]
Cost	518,914		197,095
Fair Value	522,416		201,391
Business Services
Schedule of Investments [Line Items]
Cost	381,300		209,656
Fair Value	385,317		212,035
Healthcare
Schedule of Investments [Line Items]
Cost	95,873		48,501
Fair Value	96,490		48,824
Financial Services
Schedule of Investments [Line Items]
Cost	82,832		23,573
Fair Value	83,014		23,604
Consumer Services
Schedule of Investments [Line Items]
Cost	65,139		29,992
Fair Value	65,565		30,171
Education
Schedule of Investments [Line Items]
Cost	43,074		31,499
Fair Value	44,282		32,917
Food & Beverage
Schedule of Investments [Line Items]
Cost	37,567		22,885
Fair Value	40,281		25,436
Distribution & Logistics
Schedule of Investments [Line Items]
Cost	5,466		5,499
Fair Value	5,533		5,575
Packaging
Schedule of Investments [Line Items]
Cost	1,571		547
Fair Value	1,577		513
First lien
Schedule of Investments [Line Items]
Cost	1,150,701		532,881
Fair Value	1,159,577		541,000
Second lien
Schedule of Investments [Line Items]
Cost	35,952		14,115
Fair Value	37,589		16,288
Subordinated
Schedule of Investments [Line Items]
Cost	28,962		20,895
Fair Value	31,222		21,812
Equity and other
Schedule of Investments [Line Items]
Cost	16,121		1,356
Fair Value	$ 16,087		$ 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.