Fair Value - Level III Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Total
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1,279,419	$ 1,134,669	$ 1,210,282	$ 1,111,043
Total gains or losses included in earnings		3,599
Purchases, including capitalized PIK and revolver fundings	212,547	21,346	264,484	67,362
Proceeds from sales and paydowns of investments	(171,067)	(9,036)	(205,216)	(24,015)
Transfers into Level III	9,709	21,398	26,481	2,097
Transfers out of Level III	(54,515)	(19,014)	(23,502)	(8,816)
Ending balance	1,275,491	1,152,962	1,275,491	1,152,962
Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period	(860)	3,599	1,584	5,302
Total \| Net realized losses on investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	19		(573)	(11)
Total \| Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	(621)		3,535	5,302
First lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,221,457	1,090,527	1,167,465	1,067,062
Total gains or losses included in earnings		3,200
Purchases, including capitalized PIK and revolver fundings	202,182	21,346	253,872	67,362
Proceeds from sales and paydowns of investments	(153,707)	(9,036)	(184,856)	(24,015)
Transfers into Level III	9,709	19,172	26,481	0
Transfers out of Level III	(37,245)	(19,014)	(23,502)	(8,816)
Ending balance	1,242,213	1,106,195	1,242,213	1,106,195
Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period	(362)	3,200	1,677	4,613
First lien \| Net realized losses on investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	19		(573)	(11)
First lien \| Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	(202)		3,326	4,613
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	57,714	44,141	42,816	43,980
Total gains or losses included in earnings		399
Purchases, including capitalized PIK and revolver fundings	5,985	0	5,985	0
Proceeds from sales and paydowns of investments	(17,360)	0	(20,360)	0
Transfers into Level III	0	2,226	0	2,097
Transfers out of Level III	(17,270)	0	0	0
Ending balance	28,650	46,766	28,650	46,766
Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period	(498)	399	(93)	689
Second lien \| Net realized losses on investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	0		0	0
Second lien \| Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	(419)		209	689
Subordinated
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	43	1	1	1
Total gains or losses included in earnings		0
Purchases, including capitalized PIK and revolver fundings	4,380	0	4,422	0
Proceeds from sales and paydowns of investments	0	0	0	0
Transfers into Level III	0	0	0	0
Transfers out of Level III	0	0	0	0
Ending balance	4,423	1	4,423	1
Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period	0	0	0	0
Subordinated \| Net realized losses on investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	0		0	0
Subordinated \| Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	0		0	0
Equity and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	205	0	0	0
Total gains or losses included in earnings		0
Purchases, including capitalized PIK and revolver fundings	0	0	205	0
Proceeds from sales and paydowns of investments	0	0	0	0
Transfers into Level III	0	0	0	0
Transfers out of Level III	0	0	0	0
Ending balance	205	0	205	0
Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period	0	$ 0	0	0
Equity and other \| Net realized losses on investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	0		0	0
Equity and other \| Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings	$ 0		$ 0	$ 0

NMF SLF I, Inc. 10-Q