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	$ 1,385,574	[1]	$ 1,321,455
Fair Value	1,385,068	[1]	1,320,435
Software
Schedule of Investments [Line Items]
Cost	579,927		533,247
Fair Value	581,416		535,183
Business Services
Schedule of Investments [Line Items]
Cost	299,836		301,090
Fair Value	301,500		302,599
Healthcare
Schedule of Investments [Line Items]
Cost	213,961		208,815
Fair Value	209,315		204,943
Financial Services
Schedule of Investments [Line Items]
Cost	96,189		97,914
Fair Value	96,956		98,558
Consumer Services
Schedule of Investments [Line Items]
Cost	66,557		51,517
Fair Value	66,959		51,576
Education
Schedule of Investments [Line Items]
Cost	42,661		34,168
Fair Value	42,555		34,468
Food & Beverage
Schedule of Investments [Line Items]
Cost	21,880		13,425
Fair Value	23,348		14,405
Consumer Products
Schedule of Investments [Line Items]
Cost	22,511		22,650
Fair Value	20,968		20,971
Packaging
Schedule of Investments [Line Items]
Cost	17,935		17,314
Fair Value	18,062		17,315
Distribution & Logistics
Schedule of Investments [Line Items]
Cost	11,733		22,626
Fair Value	11,632		21,938
Business Products
Schedule of Investments [Line Items]
Cost	6,317		6,403
Fair Value	6,312		6,404
Specialty Chemicals & Materials
Schedule of Investments [Line Items]
Cost	6,067		12,286
Fair Value	6,045		12,075
First lien
Schedule of Investments [Line Items]
Cost	1,331,247		1,261,268
Fair Value	1,331,076		1,260,387
Second lien
Schedule of Investments [Line Items]
Cost	48,594		59,103
Fair Value	48,102		58,870
Subordinated
Schedule of Investments [Line Items]
Cost	5,528		1,084
Fair Value	5,685		1,178
Equity and other
Schedule of Investments [Line Items]
Cost	205		0
Fair Value	$ 205		$ 0

[1] NMF SLF I, Inc. (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.