| Line Item | Jun 30, 2026 | Mar 31, 2026 | Dec 31, 2025 | Sep 30, 2025 | Jun 30, 2025 | Mar 31, 2025 | Dec 31, 2024 | Sep 30, 2024 | Jun 30, 2024 | Mar 31, 2024 | Dec 31, 2023 | Sep 30, 2023 | Jun 30, 2023 | Mar 31, 2023 | Dec 31, 2022 | Sep 30, 2022 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Investment income | ||||||||||||||||
| PIK interest income | ||||||||||||||||
| Non-cash dividend income | ||||||||||||||||
| Total investment income | ||||||||||||||||
| Expenses | ||||||||||||||||
| Interest and other financing expenses | ||||||||||||||||
| Management fee | ||||||||||||||||
| Incentive fee | ||||||||||||||||
| Professional fees | ||||||||||||||||
| Administrative expenses | ||||||||||||||||
| Other general and administrative expenses | ||||||||||||||||
| Total expenses | ||||||||||||||||
| Less: management and incentive fees waived (See Note 5) | ||||||||||||||||
| Net expenses | ||||||||||||||||
| Net investment income before income taxes | ||||||||||||||||
| Income tax expense (benefit) | ||||||||||||||||
| Net investment income | ||||||||||||||||
| Net realized (losses) gains: | ||||||||||||||||
| Investments | ||||||||||||||||
| Net change in unrealized appreciation (depreciation): | ||||||||||||||||
| Investments | ||||||||||||||||
| Securities purchased under collateralized agreements to resell | ||||||||||||||||
| Foreign currency | ||||||||||||||||
| Benefit (provision) for taxes | ||||||||||||||||
| Net realized and unrealized losses | ||||||||||||||||
| Net increase (decrease) in net assets resulting from operations | ||||||||||||||||
| Less: Net increase in net assets resulting from operations related to non-controlling interest in New Mountain Net Lease Corporation | ||||||||||||||||
| Net increase (decrease) in net assets resulting from operations related to New Mountain Finance Corporation | ||||||||||||||||
| Basic earnings (loss) per share (in dollars per share) | ||||||||||||||||
| Diluted earnings (loss) per share (in dollars per share) | ||||||||||||||||
| Distributions declared per share (in dollars per share) | — | |||||||||||||||
| Distributions paid per share (in dollars per share) | — |