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Borrowings - SBA-guaranteed Debentures, Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Instrument [Line Items]
SBA-guaranteed debentures outstanding	$ 300,000	$ 300,000
Upfront fees percentage	0.03435
Commitment fee percentage	0.0100
Issuance discount percentage	0.02435
SBIC I
Debt Instrument [Line Items]
Regulatory capital	$ 75,000	75,000
SBA-guaranteed debentures outstanding	150,000	150,000
SBIC II
Debt Instrument [Line Items]
Regulatory capital	75,000	75,000
SBA-guaranteed debentures outstanding	$ 150,000	$ 150,000
SBA-guaranteed debentures
Debt Instrument [Line Items]
Term of debt	10 years
SBA-guaranteed debentures | LIBOR
Debt Instrument [Line Items]
Interest rate margin	0.30%

X

- Definition

Guaranteed Debt

+ References

No definition available.

+ Details

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Namespace Prefix:	nmfc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Guaranteed Debt, Commitment Fee

+ References

No definition available.

+ Details

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Data Type:	xbrli:pureItemType
Balance Type:	na
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X

- Definition

Guaranteed Debt, Issuance Discount

+ References

No definition available.

+ Details

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Namespace Prefix:	nmfc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Guaranteed Debt, Upfront Fees

+ References

No definition available.

+ Details

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Data Type:	xbrli:pureItemType
Balance Type:	na
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X

- Definition

Small Business Association Regulation, Regulatory Capital, Actual

+ References

No definition available.

+ Details

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Namespace Prefix:	nmfc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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X

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Details

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X

- Details

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Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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