Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] NMNLC And NM CLFX LP New Mountain Net Lease Corporation And NM CLFX LP [Member] New Mountain Net Lease Corporation And NM CLFX LP Deferred offering costs Deferred Charges, Policy [Policy Text Block] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Related Party Transaction [Line Items] Related Party Transaction [Line Items] Permian Holdco 3, Inc. / Permian Trust Permian Holdco 3, Inc. And Permian Trust [Member] Permian Holdco 3, Inc. And Permian Trust Commitment fee percentage Guaranteed Debt, Commitment Fee Guaranteed Debt, Commitment Fee Income tax (benefit) expense Investment Income, Operating, Tax Expense (Benefit) Investment Company, Nonconsolidated Subsidiary [Axis] Investment Company, Nonconsolidated Subsidiary [Axis] All Award Types Award Type [Domain] Fair Value as of Grant Date Award Grant Date Fair Value Borrowings allowable per dollar of equity Debt Outstanding Allowable Per Dollar Of Equity Debt Outstanding Allowable Per Dollar Of Equity Accumulated Net Investment Income Retained Earnings, Accumulated Net Investment Income (Loss) [Member] Retained Earnings, Accumulated Net Investment Income (Loss) Number of investments on non-accrual Investment Owned, Nonaccrual, Number Of Investments Investment Owned, Nonaccrual, Number Of Investments Investment Company, Financial Highlights [Table] Investment Company, Financial Highlights [Table] $15,000 SBA-guaranteed debentures, Issued September 2018 USD 15,000 SBA Guaranteed Debentures, Issued September 2018 [Member] USD 15,000 SBA Guaranteed Debentures, Issued September 2018 Deferred tax liability Deferred Income Tax Liabilities, Net Minimum asset coverage ratio Asset Coverage, Required Ratio, Minimum Asset Coverage, Required Ratio, Minimum Investment Company, Nonconsolidated Subsidiary [Table] Investment Company, Nonconsolidated Subsidiary [Table] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Per share data: Investment Company, Per Share Data [Abstract] Investment Company, Per Share Data Net investment income Investment Company, Investment Income (Loss) Ratio Insider Trading Policies and Procedures [Line Items] Deferred tax liability Increase (Decrease) In Deferred Income Tax Liability Increase (Decrease) In Deferred Income Tax Liability Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Proceeds from sales and paydowns of investments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements ACI Group Holdings, Inc. ACI Group Holdings, Inc. [Member] ACI Group Holdings, Inc. Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Net Assets Equity [Text Block] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] DB Credit Facility DB Credit Facility [Member] DB Credit Facility Capital share activity Investment Company, Capital Share Transactions, Stock Issued [Abstract] Indirect administrative expenses Investment Company, Fee Paid Indirectly Number of portfolio companies Investment Company, Number Of Portfolio Companies Investment Company, Number Of Portfolio Companies Ownership [Axis] Ownership [Axis] Subordinated Subordinated Debt [Member] SBA Annual Charge Debt Instrument, Annual Charge, Percentage Debt Instrument, Annual Charge, Percentage Non-controlled/Non-affiliated Investment, Unaffiliated Issuer [Member] Total net increase (in dollars per share) Investment Company, Investment Income (Loss) from Operations, Per Share Preferred stock, par value $0.01 per share, 2,000,000 shares authorized, none issued Preferred Stock, Value, Issued Subsequent Event Type [Domain] Subsequent Event Type [Domain] Distributions paid Payments of Ordinary Dividends, Common Stock 2018A Unsecured Notes Unsecured Notes Effective January 2018 [Member] Unsecured Notes Effective January 2018 Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Schedule of Investments [Table] Schedule of Investments [Table] Drawn revolving credit facilities Drawn Revolving Credit Facilities [Member] Drawn Revolving Credit Facilities NM YI LLC NM YI, LLC NM YI LLC [Member] NM YI LLC Credit Facility Line of Credit [Member] Education Management II LLC Education Management II LLC [Member] Education Management II LLC Litigation Case [Axis] Litigation Case [Axis] Trading Symbol Administrative expenses Administrative Fees Expense After Reinvestment Period After Reinvestment Period [Member] After Reinvestment Period Debt Investments Debt Securities [Member] New Mountain Capital New Mountain Capital [Member] New Mountain Capital Fair Value Disclosures [Abstract] Incentive fee payable Increase (Decrease) in Incentive Fee Payable Amortization of premium on Convertible Notes Amortization of premium Amortization of Debt Discount (Premium) Number of investments held Investment Company, Number Of Investments Held Investment Company, Number Of Investments Held Net assets Equity, Attributable to Parent [Abstract] Schedule of Basic and Diluted Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Floating rates Debt Securities, Floating Interest Rate [Member] Debt Securities, Floating Interest Rate Executive Category: Executive Category [Axis] Net asset value per share of New Mountain Finance Corporation (in dollars per share) Net asset value, beginning balance (in dollars per share) Net asset value, ending balance (in dollars per share) Changes in Level III Portfolio Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Effect of foreign exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Including Disposal Group and Discontinued Operations Dividends [Domain] Dividends [Domain] Level I Fair Value, Inputs, Level 1 [Member] Principal Amount Reported Value Measurement [Member] Schedule of Properties Property, Plant and Equipment [Table Text Block] Equity Components [Axis] Equity Components [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Investment Type [Axis] Investment Type [Axis] Second lien Debt Securities, Second Lien [Member] Debt Securities, Second Lien Entity Small Business Investments [Domain] Investments [Domain] Local Phone Number Percent of Net Assets Investment Owned, Net Assets, Percentage Foreign currency Net change in unrealized depreciation on translation of assets and liabilities in foreign currencies Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Recovery of Erroneously Awarded Compensation Disclosure [Line Items] NMNLC Credit Facility II NMNLC Credit Facility II [Member] NMNLC Credit Facility II Numerator for diluted earnings per share: Net Income (Loss) Attributable to Parent, Diluted $19,000 SBA-guaranteed debentures, Issued September 2019 USD 19,000 SBA Guaranteed Debentures, Issued September 2019 [Member] USD 19,000 SBA Guaranteed Debentures, Issued September 2019 PPV Intermediate Holdings LLC PPV Intermediate Holdings LLC [Member] PPV Intermediate Holdings LLC Recorded Future, Inc. Recorded Future, Inc. [Member] Recorded Future, Inc. Investments, cost Cost Cost basis Investment Owned, Cost Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Securities Financing Transaction [Line Items] Securities Financing Transaction [Line Items] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Line of Credit Facility [Table] Line of Credit Facility [Table] LIBOR London Interbank Offered Rate (LIBOR) 1 [Member] London Interbank Offered Rate (LIBOR) 1 Schedule of Information Related to Borrowings Schedule of Long-Term Debt Instruments [Table Text Block] NM NL Holdings LP / NM GP Holdco LLC NM NL Holdings LP / NM GP Holdco LLC [Member] NM NL Holdings LP / NM GP Holdco LLC GC Waves Holdings, Inc. GC Waves Holdings, Inc. [Member] GC Waves Holdings, Inc. Unfunded commitments on revolving credit facilities Unfunded Debt Securities, Revolving Credit Facilities [Member] Unfunded Debt Securities, Revolving Credit Facilities Proceeds from paydowns of investments Proceeds from Disposition of Investment, Operating Activity Sale of stock, number of shares issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Management fee Management Fee Expense Interest expense Interest Expense, Debt Convertible Notes Convertible Debt Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Purchase amount per $1,000 principal amount Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Other expenses Other Expenses Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Award Type Award Type [Axis] Administration Agreement Administration Agreement [Member] Administration Agreement Document Quarterly Report Shares issued from the reinvestment of distributions (in shares) Stock Issued During Period, Shares, Dividend Reinvestment Plan Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Agreements Related Parties Related Party Transactions Disclosure [Text Block] Packaging Container and Packaging Sector [Member] Less: management fee waived (See Note 5) Investment Company, Contractual Fee Waived PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Recent accounting standards updates New Accounting Pronouncements, Policy [Policy Text Block] Australia AUSTRALIA Holdings Credit Facility, Broadly Syndicated Loans Holdings Credit Facility, Broadly Syndicated Loans [Member] Holdings Credit Facility, Broadly Syndicated Loans Counterparty Name [Domain] Counterparty Name [Domain] Total Coupon/Interest Rate Interest rate Interest Rate Investment Interest Rate $51,000 SBA-guaranteed debentures, Issued September 2020 USD 51,000 SBA Guaranteed Debentures, Issued September 2020 [Member] USD 51,000 SBA Guaranteed Debentures, Issued September 2020 Sale of stock, price per share (in dollars per share) Sale of Stock, Price Per Share Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Issuance discount percentage Guaranteed Debt, Issuance Discount Guaranteed Debt, Issuance Discount Tahoe Finco, LLC Tahoe Finco, LLC [Member] Tahoe Finco, LLC Security Exchange Name KAMC Holdings, Inc KAMC Holdings, Inc [Member] KAMC Holdings, Inc Coupa Holdings, LLC Coupa Holdings, LLC [Member] Coupa Holdings, LLC Employee Stock Option Employee Stock Option [Member] Investments Investment, Policy [Policy Text Block] Interest and dividend receivable Dividend and Interest Receivable Purchase amount Debt Instrument, Repurchased Face Amount Maximum High Maximum [Member] Cash equivalents Cash Equivalents, at Carrying Value Cash received for purchase of undrawn portion of revolving credit or delayed draw facilities Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Document Type Schedule of Management Fees and Incentive Fees Schedule of Related Party Transactions [Table Text Block] CRCI Longhorn Holdings, Inc. CRCI Longhorn Holdings, Inc. [Member] CRCI Longhorn Holdings, Inc. Tabular List, Table Tabular List [Table Text Block] Borrowing capacity, fifth percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Common stock repurchased Treasury Stock, Value, Acquired, Cost Method Debt to equity ratio, maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum NMFC Senior Loan Program III LLC SLP III NMFC Senior Loan Program III LLC [Member] NMFC Senior Loan Program III LLC Issuances of common stock Stock Issued During Period, Value, New Issues And Dividend Reinvestment Plan Stock Issued During Period, Value, New Issues And Dividend Reinvestment Plan Unearned dividend income Investment Owned, Unearned Dividend Income Investment Owned, Unearned Dividend Income Basis of accounting Basis of Accounting, Policy [Policy Text Block] Non-cash investment income Other Noncash Income (Expense) Market & income approach Valuation, Market And Income Approach [Member] Valuation, Market And Income Approach Principal Amount, Par Value Principal amount or par value Principal amount Investment Owned, Balance, Principal Amount Lender Cost Of Funds Rate Lender Cost Of Funds Rate [Member] Lender Cost Of Funds Rate Variable Rate [Axis] Variable Rate [Axis] Regulation Regulatory Capital Requirements For Investment Companies [Text Block] Regulatory Capital Requirements For Investment Companies Borrowing capacity, first percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Investment Company, Nonconsolidated Subsidiary [Line Items] Investment Company, Nonconsolidated Subsidiary [Line Items] Net investment income before income taxes Net investment income before income taxes Net Investment Income Title of 12(b) Security Financial Highlights Investment Company, Financial Highlights [Text Block] Interest Income Interest income Interest Income, Operating Related Party [Domain] Related Party, Type [Domain] Stock repurchase program Stockholders' Equity, Policy [Policy Text Block] Food & Beverage Food and Beverage Sector [Member] Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Annualized catch-up threshold Investment Management Agreement, Annualized Catch-Up Threshold [Member] Investment Management Agreement, Annualized Catch-Up Threshold Notorious Topco, LLC Notorious Topco, LLC [Member] Notorious Topco, LLC Distributions related to non-controlling interest in NMNLC Payments to Noncontrolling Interests Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount OA Buyer, Inc. OA Buyer, Inc. [Member] OA Buyer, Inc. Entity Tax Identification Number Borrowing capacity, fourth percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Statistical Measurement [Axis] Statistical Measurement [Axis] $4,000 SBA-guaranteed debentures, Issued September 2016 USD 4,000 SBA Guaranteed Debentures, Issued September 2016 [Member] USD 4,000 SBA Guaranteed Debentures, Issued September 2016 Kaseya Inc Kaseya Inc [Member] Kaseya Inc Distributions declared to stockholders from net investment income Distributions declared Investment Company, Dividend Distribution Entity Interactive Data Current Borrowings Long-Term Debt [Text Block] Consumer Services Consumer Services Sector [Member] Consumer Services Sector Unearned interest income Investment Owned, Unearned Interest Income Investment Owned, Unearned Interest Income Business Products Business Products Sector [Member] Business Products Sector Increases in dividend in excess (in dollars per share) Debt Instrument, Convertible, Dividends In Excess, Increase Debt Instrument, Convertible, Dividends In Excess, Increase Measure: Measure [Axis] Commitments and Contingencies Disclosure [Abstract] Name Outstanding Recovery, Individual Name Business Services Business Services Sector [Member] Business Services Sector Measurement Input Investment Company, Investment Owned, Measurement Input Entity Incorporation, State or Country Code Borrowing capacity, second percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Financing commitment letter to purchase investments Financing Commitment Letter to Purchase Investments [Member] Financing Commitment Letter to Purchase Investments Counterparty Name [Axis] Counterparty Name [Axis] Safety Borrower Holdings LLC Safety Borrower Holdings LLC [Member] Safety Borrower Holdings LLC Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Gross Additions Investments in and Advances to Affiliates, at Fair Value, Gross Additions Pioneer Buyer I, LLC Pioneer Buyer I, LLC [Member] Pioneer Buyer I, LLC Number of shares outstanding (in shares) Common stock, outstanding (in shares) Shares outstanding at end of period (in shares) Common Stock, Shares, Outstanding Net change in unrealized (depreciation) appreciation: Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net [Abstract] Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net 2018 Convertible Notes 2018 Convertible Notes [Member] 2018 Convertible Notes Total Investments [Member] PEO PEO [Member] NM DRVT LLC NM DRVT LLC [Member] NM DRVT LLC Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Sale of Stock [Axis] Sale of Stock [Axis] Net investment income Net investment income Investment Income, Operating, after Expense and Tax Maximum purchase amount Debt Instrument, Repurchase Face Amount, Maximum Debt Instrument, Repurchase Face Amount, Maximum Community Brands ParentCo, LLC Community Brands ParentCo, LLC [Member] Community Brands ParentCo, LLC Professional fees Professional Fees Debt Instrument [Axis] Investments Schedule of Investments [Text Block] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Sun Acquirer Corp. Sun Acquirer Corp. [Member] Sun Acquirer Corp. Measurement Input Type [Domain] Measurement Input Type [Domain] Black Elk Trustee Black Elk Trustee [Member] Black Elk Trustee Investments Net change in unrealized depreciation of investments Net change in unrealized appreciation (depreciation) of investments Debt and Equity Securities, Unrealized Gain (Loss) Granicus, Inc. Granicus, Inc. [Member] Granicus, Inc. Credit Facility [Axis] Credit Facility [Axis] SBA-guaranteed debentures SBA-guaranteed debentures outstanding Guaranteed Debt Guaranteed Debt Securities Financing Transaction [Table] Securities Financing Transaction [Table] Total liabilities Liabilities Current income tax expense (benefit) Current Income Tax Expense (Benefit) Repayments of Credit Facility Repayments of Long-Term Lines of Credit Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Related Party Transaction [Domain] Related Party Transaction [Domain] Canada CANADA Financial Services Financial Services Sector [Member] Consumer Products Consumer Products Sector [Member] Consumer Products Sector Subsequent Event [Line Items] Subsequent Event [Line Items] Affiliates Affiliated Entity [Member] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Idera, Inc. Idera, Inc. [Member] Idera, Inc. Calabrio, Inc. Calabrio, Inc. [Member] Calabrio, Inc. Jersey JERSEY Equity Equity Securities [Member] Supplemental disclosure of cash flow information Supplemental Cash Flow Information [Abstract] Xactly Corporation Xactly Corporation [Member] Xactly Corporation Maximum secured debt ratio Debt Instrument, Covenant, Secured Debt Ratio, Maximum Debt Instrument, Covenant, Secured Debt Ratio, Maximum Payable to affiliates/ Other liabilities Other liabilities Due to affiliates Other Liabilities Earnings per share Earnings Per Share, Policy [Policy Text Block] Benefit (provision) for taxes Benefit (provision) for taxes Gain (Loss), Investment and Derivative, Operating, Tax Expense (Benefit) Recent Accounting Standards Updates Accounting Standards Update and Change in Accounting Principle [Text Block] Percent of Total Investments at Fair Value Concentration Risk, Percentage Cash paid for purchase of investments Payment for Purchase, Investment, Operating Activity Symplr Software Intermediate Holdings, Inc. Symplr Software Intermediate Holdings, Inc. [Member] Symplr Software Intermediate Holdings, Inc. Investment, Issuer Name [Axis] Investment, Issuer Name [Axis] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Average debt outstanding Long-Term Debt, Average Amount Outstanding Total net decrease in net assets resulting from capital transactions Investment Company, Capital Share Transaction and Dividend Distribution, Increase (Decrease) Investment Company, Capital Share Transaction and Dividend Distribution, Increase (Decrease) Apptio, Inc. Apptio, Inc. [Member] Apptio, Inc. Net cash flows provided by (used in) operating activities Net Cash Provided by (Used in) Operating Activities Class of Stock [Axis] HS Purchaser, LLC / Help/Systems Holdings, Inc. HS Purchaser, LLC / Help/Systems Holdings, Inc. [Member] HS Purchaser, LLC / Help/Systems Holdings, Inc. Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Investment period Investment Company, Nonconsolidated Subsidiary, Investment Period Investment Company, Nonconsolidated Subsidiary, Investment Period Adjustment for dilutive effect of Convertible Notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities $24,000 SBA-guaranteed debentures, Issued March 2020 USD 24,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 24,000 SBA Guaranteed Debentures, Issued March 2020 Unsecured Notes Unsecured Notes outstanding balance Unsecured Debt Fixed rates Debt Securities, Fixed Interest Rate [Member] Debt Securities, Fixed Interest Rate Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Investment Company, Changes in Net Assets [Table] Investment Company, Change in Net Assets [Table] Securities purchased under collateralized agreements to resell (cost of $30,000 and $30,000, respectively) Fair value of securities purchased under collateralized agreements to resell Securities Purchased under Agreements to Resell Variable Rate Period [Axis] Variable Rate Period [Axis] Variable Rate Period SBA-guaranteed debentures Guaranteed Debt [Member] Guaranteed Debt Hong Kong HONG KONG SBIC II New Mountain Finance SBIC II, L.P. [Member] New Mountain Finance SBIC II, L.P. Non-usage fee Line of Credit Facility, Commitment Fee Amount Dividend income Dividend Income, Operating, Paid in Cash New Permian Holdco, L.L.C. New Permian Holdco, L.L.C. [Member] New Permian Holdco, L.L.C. Preferred stock, issued (in shares) Preferred Stock, Shares Issued Funded Debt Investments Funded Debt Securities [Member] Funded Debt Securities Equity and other Other Security Investments [Member] Entity Emerging Growth Company Sale of stock, remaining amount available for issuance and sale Sale Of Stock, Remaining Stock Available For Issuance, Amount Sale Of Stock, Remaining Stock Available For Issuance, Amount United Kingdom UNITED KINGDOM Deferred financing costs (net of accumulated amortization of $52,507 and $47,531, respectively) Deferred financing costs (net of accumulated amortization) Debt Issuance Costs, Net Agent fee percentage Line Of Credit Facility, Agent Fee, Percentage Line Of Credit Facility, Agent Fee, Percentage Integro Parent Inc. Integro Parent Inc. [Member] Integro Parent Inc. Investment, Identifier [Axis] Investment, Identifier [Axis] Pre-Incentive Fee Net Investment Income exceeds catch-up threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Title Trading Arrangement, Individual Title Preferred equity variable rate margin Preferred Stock, Dividend Rate, Basis Spread On Variable Rate Preferred Stock, Dividend Rate, Basis Spread On Variable Rate Common Stock Common Stock [Member] Individual: Individual [Axis] Avalara, Inc. Avalara, Inc. [Member] Avalara, Inc. Entity Address, Postal Zip Code Income Statement Location [Domain] Income Statement Location [Domain] Netherlands NETHERLANDS $41,000 SBA-guaranteed debentures, Issued March 2020 USD 41,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 41,000 SBA Guaranteed Debentures, Issued March 2020 Daxko Acquisition Corporation Daxko Acquisition Corporation [Member] Daxko Acquisition Corporation Investment Company, Changes in Net Assets [Line Items] Investment Company, Change in Net Assets [Line Items] Total net assets Beginning of the period Ending of the period Equity, Including Portion Attributable to Noncontrolling Interest Minimum Low Minimum [Member] Payment received for settlement Proceeds from Legal Settlements Pre-Incentive Fee Net Investment Income below catch-up threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold Investment Company, Nonconsolidated Subsidiary [Domain] Investment Company, Nonconsolidated Subsidiary [Domain] Total Square Feet Area of Real Estate Property Distributions paid per share (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Statement of Cash Flows [Abstract] Assets Assets [Abstract] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Quarterly hurdle rate Investment Management Agreement, Quarterly Hurdle Rate [Member] Investment Management Agreement, Quarterly Hurdle Rate Investments including delayed draw facilities Investments Including Delayed Draw Facilities [Member] Investments Including Delayed Draw Facilities Liabilities Liabilities [Abstract] Investment Company, Financial Highlights [Roll Forward] Investment Company, Financial Highlights [Roll Forward] New Benevis Topco, LLC New Benevis Topco, LLC [Member] New Benevis Topco, LLC Net cash flows (used in) provided by financing activities Net Cash Provided by (Used in) Financing Activities Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Accrual for deferred financing costs Debt Issuance Costs Incurred but Not Yet Paid Debt Issuance Costs Incurred but Not Yet Paid Brave Parent Holdings, Inc. Brave Parent Holdings, Inc. [Member] Brave Parent Holdings, Inc. Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Basic earnings per share (in dollars per share) Earnings Per Share, Basic Accounting Policies [Abstract] Regular distribution Regular Distribution [Member] Regular Distribution Sale of Stock [Domain] Sale of Stock [Domain] NM APP US LLC NM APP US LLC [Member] NM APP US LLC Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Receivable from unsettled securities sold Receivable, Investment, Sale Controlled Investment, Affiliated Issuer, Controlled [Member] Schedule of Information Related to Convertible Notes Convertible Debt [Table Text Block] Investments threshold period past due for nonaccrual status Investment Owned, Nonaccrual, Threshold Period Past Due, Investment Owned, Nonaccrual, Threshold Period Past Due, Ordinary shares dividend rate Common Stock, Dividend Rate, Percentage Common Stock, Dividend Rate, Percentage Borrowing capacity, third percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage PIK Investment, Interest Rate, Paid in Kind Value of shares reissued from repurchase program in connection with the distribution reinvestment plan Stock Issued Income taxes paid Income Taxes Paid Document Transition Report Award Timing Predetermined Award Timing Predetermined [Flag] Unsecured Notes Unsecured Debt [Member] Other Other Valuation Technique [Member] Other Valuation Technique Haven Midstream Holdings LLC Haven Midstream Holdings LLC [Member] Haven Midstream Holdings LLC Non-controlled/Affiliated Investment, Affiliated Issuer, Noncontrolled [Member] Reinvestment of distributions Stock Issued During Period, Value, Dividend Reinvestment Plan Redemption of shares paid-in-kind Investment Company, Redemption of Shares, Paid-In-Kind Investment Company, Redemption of Shares, Paid-In-Kind Information Technology Technology Sector [Member] Foundational Education Group, Inc. Foundational Education Group, Inc. [Member] Foundational Education Group, Inc. Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Collateralized agreements or repurchase financings Repurchase Agreements, Collateral, Policy [Policy Text Block] Sale of stock, issuance amount Sale Of Stock, Maximum Issuance, Amount Sale Of Stock, Maximum Issuance, Amount Outstanding balance Long-Term Debt, Gross All Trading Arrangements All Trading Arrangements [Member] Ownership percentage Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest All Adjustments to Compensation All Adjustments to Compensation [Member] Net Unrealized Appreciation (Depreciation) Retained Earnings, Net Unrealized Appreciation (Depreciation) [Member] Retained Earnings, Net Unrealized Appreciation (Depreciation) AAH Topco, LLC AAH Topco, LLC [Member] AAH Topco, LLC Compensation Amount Outstanding Recovery Compensation Amount Total return based on net asset value Investment Company, Total Return Deferred income tax provision (benefit) Deferred Income Tax Expense (Benefit) Subsequent Event Type [Axis] Subsequent Event Type [Axis] Convertible Notes Convertible Debt [Member] Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Net expenses Investment Company, Expense, after Reduction of Fee Waiver and Reimbursement Net investment income (in dollars per share) Investment Company, Investment Income (Loss), Per Share Education Education Sector [Member] Education Sector Other assets Other Assets Net realized gains (losses): Net Realized Gains (Losses) [Abstract] Net Realized Gains (Losses) Non-cash activity on investments Noncash Activity On Investments Noncash Activity On Investments Document Period End Date Adoption Date Trading Arrangement Adoption Date Distributions declared to stockholders from net investment income (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Management fee payable Management Fee Payable Income tax provision (benefit) Income Tax Expense (Benefit) Supplemental distribution Supplemental Distribution [Member] Supplemental Distribution Total expenses, before waivers/reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Fair Value Fair Value Disclosures [Text Block] NM GLCR LP NM GLCR LP [Member] NM GLCR LP Dividend Income Dividend income Dividend Income, Operating Total return based on market value Investment Company, Total Return, Market Value 2022 Convertible Notes 2022 Convertible Notes [Member] 2022 Convertible Notes Equity [Abstract] NM KRLN LLC NM KRLN LLC [Member] NM KRLN LLC Realized capital gains Accrued capital gains incentive fees Investment Management Agreement , Realized Capital Gains [Member] Investment Management Agreement , Realized Capital Gains Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Investments excluding drawn revolvers Investments Excluding Drawn Revolvers [Member] Investments Excluding Drawn Revolvers MRI Software LLC MRI Software LLC [Member] MRI Software LLC NMFC Senior Loan Program IV LLC SLP IV NMFC Senior Loan Program IV LLC [Member] NMFC Senior Loan Program IV LLC FS WhiteWater Borrower, LLC FS WhiteWater Borrower, LLC [Member] FS WhiteWater Borrower, LLC Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Fair Value Property, Plant, and Equipment, Fair Value Disclosure VT Topco, Inc. VT Topco, Inc. [Member] VT Topco, Inc. Upfront fees percentage Guaranteed Debt, Upfront Fees Guaranteed Debt, Upfront Fees Credit Facility [Domain] Credit Facility [Domain] $13,000 SBA-guaranteed debentures, Issued September 2017 USD 13,000 SBA Guaranteed Debentures, Issued September 2017 [Member] USD 13,000 SBA Guaranteed Debentures, Issued September 2017 Interest rate margin Debt Instrument, Basis Spread on Variable Rate Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Damages sought Loss Contingency, Damages Sought, Value Proceeds from Unsecured Notes Proceeds from Issuance of Unsecured Debt NMC Crimson Holdings, Inc. NMC Crimson Holdings, Inc. [Member] NMC Crimson Holdings, Inc. Other commitments Other Commitment Auctane Inc. (fka Stamps.com Inc.) Auctane Inc. (fka Stamps.com Inc.) [Member] Auctane Inc. (fka Stamps.com Inc.) Payable to affiliates Increase (Decrease) in Due to Affiliates EBITDA multiple Measurement Input, EBITDA Multiple [Member] During Reinvestment Period During Reinvestment Period [Member] During Reinvestment Period Paid in Capital in Excess of Par Additional Paid-in Capital [Member] NMFC Credit Facility NMFC Credit Facility [Member] NMFC Credit Facility Repayment of Unsecured Notes Repayment of Unsecured Notes Repayments of Unsecured Debt TigerConnect, Inc. TigerConnect, Inc. [Member] TigerConnect, Inc. Subsequent Events Subsequent Events [Text Block] Amortization of deferred financing costs Amortization of financing costs Amortization of Debt Issuance Costs United States UNITED STATES Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Investment Funds (includes investments in joint ventures) Investment Funds, Including Investments In Joint Ventures [Member] Investment Funds, Including Investments In Joint Ventures Weighted average interest rate Long-Term Debt, Weighted Average Interest Rate, over Time Purchases, including capitalized PIK and revolver fundings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Interest and other financing expenses Interest Expense, Policy [Policy Text Block] SkyKnight Alpha SkyKnight Income Alpha, LLC [Member] SkyKnight Income Alpha, LLC LIBOR, SONIA or EURIBOR London Interbank Offered Rate (LIBOR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) [Member] London Interbank Offered Rate (LIBOR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) Equity Component [Domain] Equity Component [Domain] Number of incentive fee components Management And Service Fees, Incentive Rate, Number Of Components Management And Service Fees, Incentive Rate, Number Of Components Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Borrowings Long-Term Debt, Unclassified [Abstract] Entity Current Reporting Status Concentration Risk Type [Domain] Concentration Risk Type [Domain] Drawn revolvers Drawn Revolvers [Member] Drawn Revolvers Ratio to average net assets: Investment Company, Ratio To Average Net Assets [Abstract] Investment Company, Ratio To Average Net Assets Fair Value Estimate of Fair Value Measurement [Member] Dividends [Axis] Dividends [Axis] Investments at fair value that are non-qualifying assets as a percentage of total assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets NM CLFX LP NM CLFX LP [Member] NM CLFX LP Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Credit facility Credit facility outstanding balance Long-Term Line of Credit Distribution & Logistics Transportation Sector [Member] Variable Rate [Domain] Variable Rate [Domain] Less: Net (increase) decrease in net assets resulting from operations related to non-controlling interest in New Mountain Net Lease Corporation Net Income (Loss) Attributable to Noncontrolling Interest Pay vs Performance Disclosure [Line Items] PhyNet Dermatology LLC PhyNet Dermatology LLC [Member] PhyNet Dermatology LLC Statistical Measurement [Domain] Statistical Measurement [Domain] Schedule of Principal Amounts and Fair Values of Borrowings Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table Text Block] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Number of collateralized agreements Number Of Collateralized Agreements Number Of Collateralized Agreements Statement of Stockholders' Equity [Abstract] UniTek Global Services, Inc. UniTek Global Services, Inc. [Member] UniTek Global Services, Inc. Revolving Credit Facility Revolving Credit Facility [Member] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Foreside Financial Group, LLC Foreside Financial Group, LLC [Member] Foreside Financial Group, LLC Transfers out of Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Restatement Determination Date: Restatement Determination Date [Axis] CG Group Holdings, LLC CG Group Holdings, LLC [Member] CG Group Holdings, LLC Revenue multiple Measurement Input, Revenue Multiple [Member] Commitments and contingencies (See Note 9) Commitments and Contingencies Accumulated Net Realized (Losses) Gains Retained Earnings, Accumulated Net Realized Gain (Loss) [Member] Retained Earnings, Accumulated Net Realized Gain (Loss) Net proceeds from shares sold Net proceeds from offerings of common stock Proceeds from Issuance of Common Stock Geographical [Axis] Geographical [Axis] Industry Sector [Domain] Industry Sector [Domain] $13,950 SBA-guaranteed debentures, Issued March 2016 USD 13,950 SBA Guaranteed Debentures, Issued March 2016 [Member] USD 13,950 SBA Guaranteed Debentures, Issued March 2016 Less: expenses waived and reimbursed (See Note 5) Fees waived by the administrator Investment Company, Voluntary Fee Waived Revenue recognition Revenue [Policy Text Block] Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) QID TRH Holdings, LLC QID TRH Holdings, LLC [Member] QID TRH Holdings, LLC Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Variable Rate Period [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] NMFC Senior Loan Program IV LLC , at Cost NMFC Senior Loan Program IV LLC, at Cost [Member] NMFC Senior Loan Program IV LLC, at Cost Number of fee components Management And Service Fees, Number Of Components Management And Service Fees, Number Of Components PEO Total Compensation Amount PEO Total Compensation Amount Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Distribution payable Distribution Payable Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Level III Fair Value, Inputs, Level 3 [Member] Increase in interest rate margin during an event of default Debt Instrument, Basis Spread On Variable Rate, Increase During Event Of Default Debt Instrument, Basis Spread On Variable Rate, Increase During Event Of Default Net change in unrealized (depreciation) appreciation Debt and Equity Securities, Unrealized Gain (Loss) [Member] Debt and Equity Securities, Unrealized Gain (Loss) Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Borrowings Long-Term Debt, Fair Value Net increase in net assets resulting from operations related to New Mountain Finance Corporation Numerator for basic earnings per share Numerator for increase in net assets per share Net Income (Loss) Issuances of common stock (in shares) Stock Issued During Period, Shares, New Issues And Dividend Reinvestment Plan Stock Issued During Period, Shares, New Issues And Dividend Reinvestment Plan Foreign securities Foreign Currency Transactions and Translations Policy [Policy Text Block] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Company Selected Measure Name Company Selected Measure Name Investments at fair value Investments, At Fair Value [Member] Investments, At Fair Value Investment, Issuer Affiliation [Domain] Investment, Issuer Affiliation [Domain] Total expenses, net of waivers/reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Repayment of Convertible Notes Repayment of Convertible Notes Repayments of Convertible Debt Deca Dental Holdings LLC Deca Dental Holdings LLC [Member] Deca Dental Holdings LLC Interest Rate Type Interest Rate Type Risk [Member] Interest Rate Type Risk Net (decrease) increase in net assets Stockholders' Equity, Period Increase (Decrease) Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Common Class B Common Class B [Member] Investments Net realized gains on investments Net realized losses on investments Debt and Equity Securities, Realized Gain (Loss) Other Commitments [Table] Other Commitments [Table] GS Acquisitionco, Inc GS Acquisitionco, Inc [Member] GS Acquisitionco, Inc SkyKnight II SkyKnight Income II, LLC [Member] SkyKnight Income II, LLC Name Measure Name Available borrowing capacity Line of Credit Facility, Current Borrowing Capacity Name Forgone Recovery, Individual Name Specialtycare, Inc. Specialtycare, Inc. [Member] Specialtycare, Inc. PIK interest income Investment income, interest, paid-in-kind Reversal of previously recorded PIK interest Interest Income, Operating, Paid in Kind Management fees waived Investment Company, Contractual Fee Waived, Management Fee Investment Company, Contractual Fee Waived, Management Fee Measurement Basis [Axis] Measurement Basis [Axis] Underlying Securities Award Underlying Securities Amount Trinity Air Consultants Holdings Corporation Trinity Air Consultants Holdings Corporation [Member] Trinity Air Consultants Holdings Corporation Permian Trust Permian Trust [Member] Permian Trust Fair Value Measurement [Domain] Fair Value Measurement [Domain] Distributions declared per share (in dollars per share) Common Stock, Dividends, Per Share, Declared Warrants Warrant [Member] Foreign currency Net realized gains on translation of assets and liabilities in foreign currencies Realized Gain (Loss), Foreign Currency Transaction, before Tax Schedule of Summarized Financial Information for Nonconsolidated Entities Investment Company, Nonconsolidated Subsidiary, Summarized Financial Information [Table Text Block] Term of debt Debt Instrument, Term Offering costs paid Offering expenses Payments of Stock Issuance Costs Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] DOCS, MSO, LLC DOCS, MSO, LLC [Member] DOCS, MSO, LLC Deferred financing costs paid Payments of Debt Issuance Costs Income Statement Location [Axis] Income Statement Location [Axis] Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] Incentive fee percentage Management and Service Fees, Incentive Rate Cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Offering costs Offering costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Conversion premium Debt Instrument, Convertible Conversion Premium Debt Instrument, Convertible Conversion Premium Decrease (Increase) in operating assets: Increase (Decrease) in Operating Assets [Abstract] Schedule of Net Assets Investment Company, Change in Net Assets [Table Text Block] Accrual for offering costs Stock Issuance Costs Incurred but Not Yet Paid Stock Issuance Costs Incurred but Not Yet Paid Other Performance Measure, Amount Other Performance Measure, Amount Adjustments to reconcile net increase in net assets resulting from operations to net cash (used in) provided by operating activities: Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities [Abstract] Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities CentralSquare Technologies, LLC CentralSquare Technologies, LLC [Member] CentralSquare Technologies, LLC Specialty Chemicals & Materials Chemicals Sector [Member] American Achievement Corporation (aka AAC Holding Corp.) American Achievement Corporation (aka AAC Holding Corp.) [Member] American Achievement Corporation (aka AAC Holding Corp.) NM JRA LLC NM JRA LLC [Member] NM JRA LLC Interest and other financing expenses Interest Expense Trading Arrangement: Trading Arrangement [Axis] Interest income (excluding Payment-in-kind ("PIK") interest income) Interest Income, Operating, Paid in Cash Use of estimates Use of Estimates, Policy [Policy Text Block] Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Entity File Number Accretion effect (in dollars per share) Common Stock, Accretion Effect Per Share Common Stock, Accretion Effect Per Share Discount rate Measurement Input, Discount Rate [Member] Spread Investment, Basis Spread, Variable Rate Entity Shell Company Restatement Determination Date Restatement Determination Date Ansira Holdings, Inc. Ansira Holdings, Inc. [Member] Ansira Holdings, Inc. Sierra Hamilton Holdings Corporation Sierra Hamilton Holdings Corporation [Member] Sierra Hamilton Holdings Corporation Increase (decrease) in net assets resulting from operations: Net Income (Loss), Including Portion Attributable to Noncontrolling Interest [Abstract] Adjustment for interest on Convertible Notes and incentive fees, net Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities Pre-Incentive Fee Net Investment Income Incentive fee, excluding accrued capital gains incentive fees Investment Management Agreement, Pre-Incentive Fee Net Investment Income [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Other liabilities Increase (Decrease) in Other Operating Liabilities Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents at the beginning of the period Cash and cash equivalents at the end of the period Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents Asset Class [Axis] Asset Class [Axis] Common stock, issued (in shares) Beginning of the period (in shares) End of the period (in shares) Common Stock, Shares, Issued 2017A Unsecured Notes Unsecured Notes Effective June 2017 [Member] Unsecured Notes Effective June 2017 Edmentum Inc. Edmentum Inc. [Member] Edmentum Inc. Net borrowings Long-Term Debt Sale of stock, consideration received Sale of stock, proceeds received Sale of Stock, Consideration Received on Transaction Regulatory capital Small Business Association Regulation, Regulatory Capital, Actual Small Business Association Regulation, Regulatory Capital, Actual Groundworks, LLC Groundworks, LLC [Member] Groundworks, LLC Reference Rates Yield Risk [Member] Reference Rates Yield Risk Net realized gains (losses) on investments and foreign currency Net Realized Gains (Losses) Realized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Entity Address, Address Line One CFS Management, LLC CFS Management, LLC [Member] CFS Management, LLC Associations, Inc. Associations, Inc. [Member] Associations, Inc. Net increase in shares outstanding (in shares) Stock Issued During Period, Shares, Period Increase (Decrease) Subsequent Event [Table] Subsequent Event [Table] DCA Investment Holding, LLC DCA Investment Holding, LLC [Member] DCA Investment Holding, LLC Net increase (decrease) in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Subsequent Event Subsequent Event [Member] Interest rate Related Party Transaction, Rate Principal Amount, Shares (in shares) Principal amount (in shares) Investment Owned, Balance, Shares Software Software Sector [Member] Software Sector Income Statement [Abstract] Schedule of Investments [Line Items] Schedule of Investments [Line Items] Wealth Enhancement Group, LLC Wealth Enhancement Group, LLC [Member] Wealth Enhancement Group, LLC Shares sold (in shares) Stock Issued During Period, Shares, New Issues Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) / Eagle Infrastructure Super HoldCo, LLC Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.), Eagle Infrastructure Super HoldCo, LLC [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.), Eagle Infrastructure Super HoldCo, LLC Reduced base management fee percentage Management And Service Fees, Base Rate, Target Under Waiver Agreement Management And Service Fees, Base Rate, Target Under Waiver Agreement Investment extension period Investment Company, Nonconsolidated Subsidiary, Investment Extension Period Investment Company, Nonconsolidated Subsidiary, Investment Extension Period Investment Type Investment Type Concentration Risk [Member] Investment Type Concentration Risk Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Holdings Credit Facility Holdings Credit Facility [Member] Holdings Credit Facility QID NGL, LLC QID NGL, LLC [Member] QID NGL, LLC Cash and other assets Cash And Other Assets Cash And Other Assets Variable Rate Component [Domain] Variable Rate Component [Domain] Variable Rate Component [Domain] Earnings per share—diluted Earnings Per Share, Diluted [Abstract] Incentive fee Incentive Fee Expense Other Commitments [Domain] Other Commitments [Domain] Investment Company, Financial Highlights [Line Items] Investment Company, Financial Highlights [Line Items] Industry Sector [Axis] Industry Sector [Axis] Financial Instrument [Axis] Financial Instrument [Axis] Securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Total Shareholder Return Amount Total Shareholder Return Amount Entity Common Stock, Shares Outstanding Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Deferred financing costs, accumulated amortization Accumulated Amortization, Debt Issuance Costs New Mountain Finance Corporation Total Net Assets of NMFC Parent [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Holdings Credit Facility, Excluding Broadly Syndicated Loans Holdings Credit Facility, Excluding Broadly Syndicated Loans [Member] Holdings Credit Facility, Excluding Broadly Syndicated Loans Accumulated undistributed earnings Retained Earnings (Accumulated Deficit) Covenant-Lite Loans Risk [Member] Covenant-Lite Loans Risk Increase (decrease) in operating liabilities: Increase (Decrease) in Operating Liabilities [Abstract] Transfers into Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Face amount Debenture Amount Debt Instrument, Face Amount Convey Health Solutions, Inc. Convey Health Solutions, Inc. [Member] Convey Health Solutions, Inc. Other Commitments [Line Items] Other Commitments [Line Items] AmeriVet Partners Management, Inc. AmeriVet Partners Management, Inc. [Member] AmeriVet Partners Management, Inc. Entity Address, State or Province Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] 2018B Unsecured Notes Unsecured Notes Effective July 2018 [Member] Unsecured Notes Effective July 2018 Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Total management fee Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Haven Midstream LLC / Haven Midstream Holdings LLC / QID TRH Holdings LLC Haven Midstream LLC And Haven Midstream Holdings LLC And QID TRH Holdings LLC [Member] Haven Midstream LLC And Haven Midstream Holdings LLC And QID TRH Holdings LLC Interest payable Increase (Decrease) in Interest Payable, Net Conversion price (in dollars per share) Debt Instrument, Convertible, Conversion Price Conversion rate Debt Instrument, Convertible, Conversion Ratio Portfolio turnover Investment Company, Portfolio Turnover Earnings per share—basic Earnings Per Share, Basic [Abstract] Interest and dividend receivable Increase (Decrease) in Interest and Dividends Receivable Asset Class [Domain] Asset Class [Domain] Class of Stock [Domain] Class of Stock [Domain] Net change in unrealized depreciation of investments and foreign currency Net Change In Unrealized Appreciation (Depreciation) Unrealized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax IMO Investor Holdings, Inc. IMO Investor Holdings, Inc. [Member] IMO Investor Holdings, Inc. Investment Company [Abstract] Investments at fair value Fair Value Fair Value, beginning balance Fair Value, ending balance Fair value Investment Owned, Fair Value Quarterly catch-up threshold Investment Management Agreement, Quarterly Catch-Up Threshold [Member] Investment Management Agreement, Quarterly Catch-Up Threshold Contributions related to non-controlling interest in NMNLC Noncontrolling Interest, Increase from Subsidiary Equity Issuance Formation and Business Purpose Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount All Executive Categories All Executive Categories [Member] $28,795 SBA-guaranteed Debentures, Issued September 2015 USD 28,795 SBA Guaranteed Debentures, Issued September 2015 [Member] USD 28,795 SBA Guaranteed Debentures, Issued September 2015 Annual increase in interest rate margin after revolving period Debt Instrument, Basis Spread On Variable Rate, Annual Increase After Revolving Period Debt Instrument, Basis Spread On Variable Rate, Annual Increase After Revolving Period Debt Disclosure [Abstract] Earnings Per Share [Abstract] Investment Management Agreement Investment Management Agreement [Member] Investment Management Agreement YLG Holdings, Inc. YLG Holdings, Inc. [Member] YLG Holdings, Inc. Common stock, par value $0.01 per share, 200,000,000 shares authorized, and 101,439,649 and 100,937,026 shares issued and outstanding, respectively Common Stock, Value, Issued Redemption as a percentage of principal Debt Instrument, Redemption Price, Percentage Non-cash dividend income Investment income, dividend, paid-in-kind Dividend Income, Operating, Paid in Kind Organization, Consolidation and Presentation of Financial Statements [Abstract] National HME, Inc./NHME Holdings Corp. National HME, Inc. And NHME Holdings Corp. [Member] National HME, Inc. And NHME Holdings Corp. Unsecured Management Company Revolver Unsecured Management Company Revolver [Member] Unsecured Management Company Revolver Interest payable Interest Payable Unfunded commitments on bridge facilities Bridge financing commitments Unfunded Debt Securities, Bridge Facilities [Member] Unfunded Debt Securities, Bridge Facilities Energy Energy Sector [Member] All Individuals All Individuals [Member] Litigation Case [Domain] Litigation Case [Domain] Entity Filer Category Non-cash operating activities: Non Cash Operating And Investing Activities [Abstract] Non Cash Operating And Investing Activities Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount NM NL Holdings, L.P. NM NL Holdings, L.P. [Member] NM NL Holdings, L.P. Statement [Table] Statement [Table] Current Fiscal Year End Date National HME, Inc. National HME, Inc. [Member] National HME, Inc. Stock repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Coyote Buyer, LLC Coyote Buyer, LLC [Member] Coyote Buyer, LLC KPSKY Acquisition Inc. KPSKY Acquisition Inc. [Member] KPSKY Acquisition Inc. USRP Holdings, Inc. USRP Holdings, Inc. [Member] USRP Holdings, Inc. PEO Name PEO Name Deferred financing costs Debt, Policy [Policy Text Block] Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Base Rate Base Rate [Member] Incentive fee payable Incentive Fee Payable Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Repurchase amount Investment Owned, Balance, Repurchase Amount Investment Owned, Balance, Repurchase Amount SOFR Secured Overnight Financing Rate (SOFR) [Member] Secured Overnight Financing Rate (SOFR) Unfunded Debt Investments Unfunded Debt Securities [Member] Unfunded Debt Securities Other income Other Income Reversal of previously recorded other income Other Income Settlement amount awarded to other party Litigation Settlement, Amount Awarded to Other Party Stated interest rate Debt Instrument, Interest Rate, Stated Percentage New Permian Holdco, Inc. / New Permian Holdco, L.L.C. New Permian Holdco, Inc. And New Permian Holdco, L.L.C. [Member] New Permian Holdco, Inc. And New Permian Holdco, L.L.C. Statement of Financial Position [Abstract] Bluefin Holding, LLC Bluefin Holding, LLC [Member] Bluefin Holding, LLC Total net assets of New Mountain Finance Corporation Equity, Attributable to Parent Schedule of Fair Value Measurement Inputs and Valuation Techniques Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Funded Investments Funded Securities [Member] Funded Securities OEConnection LLC OEConnection LLC [Member] OEConnection LLC Concentration Risk Type [Axis] Concentration Risk Type [Axis] Subsequent Events [Abstract] EBITDA as defined for middle market business Investment Company, Investment Owned, Investment Objective, Middle Market Business, EBITDA Investment Company, Investment Owned, Investment Objective, Middle Market Business, EBITDA Payable for unsettled securities purchased Payable, Investment, Purchase Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair value Fair Value Measurement, Policy [Policy Text Block] Base management fee percentage Management and Service Fees, Base Rate Level II Fair Value, Inputs, Level 2 [Member] TVG-Edmentum Holdings, LLC / Edmentum Ultimate Holdings, LLC TVG-Edmentum Holdings, LLC And Edmentum Ultimate Holdings, LLC [Member] TVG-Edmentum Holdings, LLC And Edmentum Ultimate Holdings, LLC Interest Rate Debt Instrument, Interest Rate, Effective Percentage Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] First lien Debt Securities, First Lien [Member] Debt Securities, First Lien Document Fiscal Period Focus Maximum amount of borrowings available Line of Credit Facility, Remaining Borrowing Capacity Infogain Corporation Infogain Corporation [Member] Infogain Corporation Net realized and unrealized losses (in dollars per share) Investment Company, Gain (Loss) on Investment, Per Share Weighted average Weighted Average Weighted Average [Member] City Area Code Earnings Per Share Earnings Per Share [Text Block] SBIC I New Mountain Finance SBIC, L.P. [Member] New Mountain Finance SBIC, L.P. Document Fiscal Year Focus Unfunded commitments on delayed draws or other future funding commitments Other future funding commitments Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments [Member] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments Geographical [Domain] Geographical [Domain] Investment, Issuer Name [Domain] Investment, Issuer Name [Domain] Unfunded Investments Unfunded Securities [Member] Unfunded Securities Shares redeemed for a promissory note (in shares) Investment Owned, Shares Redeemed Investment Owned, Shares Redeemed Exercise Price Award Exercise Price 2022A Unsecured Notes Unsecured Notes Effective June 2022 [Member] Unsecured Notes Effective June 2022 Reimbursements to administrators as a percentage of gross assets Investment Company, Fee Reimbursement To Gross Assets Investment Company, Fee Reimbursement To Gross Assets Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Nonrelated parties Nonrelated Party [Member] Net increase in net assets resulting from operations Net increase in net assets resulting from operations Net increase (decrease) in net assets resulting from operations Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net realized gains (losses) on investments Debt and Equity Securities, Realized Gain (Loss) [Member] Debt and Equity Securities, Realized Gain (Loss) Agreement term Related Party Transaction, Term Related Party Transaction, Term Strain On Banking System Risk [Member] Strain On Banking System Risk Schedule of Portfolio Investments by Level in the Fair Value Hierarchy Fair Value, Assets Measured on Recurring Basis [Table Text Block] NM GP Holdco, LLC NM GP Holdco, LLC [Member] NM GP Holdco, LLC Amortization of purchase discount Accretion (Amortization) of Discounts and Premiums, Investments Total liabilities and net assets Liabilities and Equity Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Ownership [Domain] Ownership [Domain] Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Payable for unsettled securities purchased Increase (Decrease) in Payable for Investment Purchased New Benevis Topco, LLC / New Benevis Holdco, Inc. New Benevis Topco, LLC And New Benevis Holdco, Inc. [Member] New Benevis Topco, LLC And New Benevis Holdco, Inc. Net realized and unrealized losses Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax Arrangement Duration Trading Arrangement Duration Members' capital Members' Equity Total expenses Investment Income, Investment Expense Entity Address, City or Town Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Industry Type Industry Concentration Risk [Member] Industry Concentration Risk TMK Hawk Parent, Corp. TMK Hawk Parent, Corp. [Member] TMK Hawk Parent, Corp. Securities purchased under collateralized agreements to resell, cost Cost basis of securities purchased under collateralized agreements to resell Securities Purchased Under Agreements To Resell, Cost Securities Purchased Under Agreements To Resell, Cost Bullhorn, Inc. Bullhorn, Inc. [Member] Bullhorn, Inc. Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) SOFR , SONIA or EURIBOR Secured Overnight Financing Rate (SOFR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) [Member] Secured Overnight Financing Rate (SOFR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) Termination Date Trading Arrangement Termination Date Common stock, authorized (in shares) Common Stock, Shares Authorized Schedule of Investments [Abstract] Contributions related to non-controlling interest in NMNLC Proceeds from Noncontrolling Interests Summary of Investment Holdings [Line Items] Summary of Investment Holdings [Line Items] Capital transactions Investment Company, Capital Share Transaction, Increase (Decrease) [Abstract] Gross Redemptions Investments in and Advances to Affiliates, at Fair Value, Gross Reductions Related Party Transactions [Abstract] Measurement Input Type [Axis] Measurement Input Type [Axis] Schedule of Investments Investment Holdings, Schedule of Investments [Table Text Block] Award Timing Disclosures [Line Items] Management fee payable Increase (Decrease) in Management Fee Payable 2019A Unsecured Notes Unsecured Notes Effective April 2019 [Member] Unsecured Notes Effective April 2019 $15,255 SBA-guaranteed debentures, Issued March 2018 USD 15,255 SBA Guaranteed Debentures, Issued March 2018 [Member] USD 15,255 SBA Guaranteed Debentures, Issued March 2018 Other Commitments [Axis] Other Commitments [Axis] Paid in capital in excess of par Additional Paid in Capital NM APP CANADA CORP NM APP Canada Corp NM APP Canada Corp [Member] NM APP Canada Corp Incomes taxes Income Tax, Policy [Policy Text Block] Insider Trading Arrangements [Line Items] Oranje Holdco, Inc. Oranje Holdco, Inc. [Member] Oranje Holdco, Inc. Other general and administrative expenses Other General and Administrative Expense Related Party [Axis] Related Party, Type [Axis] Accounting Changes and Error Corrections [Abstract] Entity Registrant Name Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Award Timing Method Award Timing Method [Text Block] Other assets Increase (Decrease) in Other Operating Assets Adjustment to Compensation, Amount Adjustment to Compensation Amount Net Lease Real Estate Sector [Member] Knockout Intermediate Holdings I Inc. Knockout Intermediate Holdings I Inc. [Member] Knockout Intermediate Holdings I Inc. Non-controlling interest in New Mountain Net Lease Corporation Equity, Attributable to Noncontrolling Interest Investment, Issuer Affiliation [Axis] Investment, Issuer Affiliation [Axis] Distributions Investment Company, Distribution [Policy Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Preferred equity dividend rate Preferred Stock, Dividend Rate, Percentage Annualized hurdle rate Investment Management Agreement, Annualized Hurdle Rate [Member] Investment Management Agreement, Annualized Hurdle Rate Entity Central Index Key Non-cash financing activities: Noncash Financing Items [Abstract] Noncash Financing Items Total gains or losses included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Dividend reinvestment plan, share price percentage, threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component $37,500 SBA-guaranteed Debentures, Issued March 2015 USD 37,500 SBA Guaranteed Debentures, Issued March 2015 [Member] USD 37,500 SBA Guaranteed Debentures, Issued March 2015 Name Trading Arrangement, Individual Name Net proceeds from shares sold Stock Issued During Period, Value, New Issues Entity [Domain] Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Amendment Flag ATM Offering At the Market Offering [Member] At the Market Offering Legal Entity [Axis] Investments in Majority-owned Subsidiaries Investment, Affiliated Issuer, Controlled, Majority-Owned [Member] Cardinal Parent, Inc. Cardinal Parent, Inc. [Member] Cardinal Parent, Inc. Cash interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities Per share market value (in dollars per share) Investment Company, Market Value, Per Share IG Investments Holdings, LLC IG Investments Holdings, LLC [Member] IG Investments Holdings, LLC Pye-Barker Fire & Safety, LLC Pye-Barker Fire & Safety, LLC [Member] Pye-Barker Fire & Safety, LLC Weighted average shares of common stock outstanding - diluted (in shares) Denominator for diluted weighted average share (in shares) Weighted Average Number of Shares Outstanding, Diluted Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] New Benevis Holdco, Inc. New Benevis Holdco, Inc. [Member] New Benevis Holdco, Inc. Expenses Operating Expenses [Abstract] Company Selected Measure Amount Company Selected Measure Amount Education Management Corporation Education Management Corporation [Member] Education Management Corporation Total assets Assets Name Awards Close in Time to MNPI Disclosures, Individual Name Tennessee Bidco Limited Tennessee Bidco Limited [Member] Tennessee Bidco Limited Fortis Solutions Group, LLC Fortis Solutions Group, LLC [Member] Fortis Solutions Group, LLC Proceeds from Convertible Notes Proceeds from Convertible Debt Effective interest rate Debt Instrument, Interest Rate During Period Non-NEOs Non-NEOs [Member] $37,500 SBA-guaranteed Debentures, Issued September 2015 USD 37,500 SBA Guaranteed Debentures, Issued September 2015 [Member] USD 37,500 SBA Guaranteed Debentures, Issued September 2015 Prime Rate Prime Rate [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Total investment income Gross Investment Income, Operating Non-PEO NEO Non-PEO NEO [Member] Adjustment to Compensation: Adjustment to Compensation [Axis] Healthcare Healthcare Sector [Member] 2021A Unsecured Notes Unsecured Notes Effective January 2021 [Member] Unsecured Notes Effective January 2021 Average net assets for the period Average Net Assets Average Net Assets Debt Instrument [Line Items] Debt Instrument [Line Items] Weighted average shares of common stock outstanding - basic (in shares) Denominator for basic weighted average share (in shares) Average weighted shares outstanding for the period - basic (in shares) Weighted Average Number of Shares Outstanding, Basic Syndigo LLC Syndigo LLC [Member] Syndigo LLC iCIMS, Inc. iCIMS, Inc. [Member] iCIMS, Inc. Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] New Mountain Net Lease Corporation NMNLC New Mountain Net Lease Corporation [Member] New Mountain Net Lease Corporation Investment income Revenues [Abstract] Assets under management Assets under Management, Carrying Amount CHA Holdings, Inc. CHA Holdings, Inc. [Member] CHA Holdings, Inc. Related Party Transaction [Axis] Related Party Transaction [Axis] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Commitment fee percentage on unused capacity Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Statement [Line Items] Statement [Line Items] Diligent Corporation Diligent Corporation [Member] Diligent Corporation Proceeds from Credit Facility Proceeds from Long-Term Lines of Credit Summary of Investment Holdings [Table] Summary of Investment Holdings [Table] Non-Controlling Interest in NMNLC Noncontrolling Interest [Member]