NMNLC And NM CLFX LP New Mountain Net Lease Corporation And NM CLFX LP [Member] New Mountain Net Lease Corporation And NM CLFX LP Investments Investment, Policy [Policy Text Block] Non-controlled/Non-affiliated Investment, Unaffiliated Issuer [Member] Award Timing Predetermined Award Timing Predetermined [Flag] Commitment fee percentage Guaranteed Debt, Commitment Fee Guaranteed Debt, Commitment Fee Securities Act File Number Paid in Capital in Excess of Par Additional Paid-in Capital [Member] Borrowings allowable per dollar of equity Debt Outstanding Allowable Per Dollar Of Equity Debt Outstanding Allowable Per Dollar Of Equity Tabular List, Table Tabular List [Table Text Block] Net realized and unrealized losses (in dollars per share) Investment Company, Gain (Loss) on Investment, Per Share Investments Schedule of Investments [Text Block] 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 Recent accounting standards updates New Accounting Pronouncements, Policy [Policy Text Block] $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 Schedule of Management Fees and Incentive Fees Incurred by the Company Schedule of Related Party Transactions [Table Text Block] 6.200% Notes due 2027 Six Point Two Percentage Notes Due Two Thousand Twenty Seven [Member] Six Point Two Percentage Notes Due Two Thousand Twenty Seven Minimum asset coverage ratio Asset Coverage, Required Ratio, Minimum Asset Coverage, Required Ratio, Minimum Kele Holdco, Inc. Kele Holdco, Inc. [Member] Kele Holdco, Inc. Total return based on market value Investment Company, Total Return, Market Value Per share data: Investment Company, Per Share Data [Abstract] Investment Company, Per Share Data Measurement Basis [Axis] Measurement Basis [Axis] Other expenses Other Expenses Adjustment to Compensation: Adjustment to Compensation [Axis] Agreements Related Parties Related Party Transactions Disclosure [Text Block] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Deferred tax liability Increase (Decrease) In Deferred Income Tax Liability Increase (Decrease) In Deferred Income Tax Liability 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. Investment Company, Nonconsolidated Subsidiary [Axis] Investment Company, Nonconsolidated Subsidiary [Axis] Repayment of DB Credit Facility DB Credit Facility DB Credit Facility [Member] DB Credit Facility Atlas AU Bidco Pty Ltd Atlas AU Bidco Pty Ltd [Member] Atlas AU Bidco Pty Ltd Number of portfolio companies Investment Company, Number Of Portfolio Companies Investment Company, Number Of Portfolio Companies Current income tax expense Current Income Tax Expense (Benefit) Total square feet Area of Real Estate Property Non-Extending Lenders Non-Extending Lenders [Member] Non-Extending Lenders Adjustment to Compensation, Amount Adjustment to Compensation Amount Percent of Total Investments at Fair Value Concentration Risk, Percentage SBA Annual Charge Debt Instrument, Annual Charge, Percentage Debt Instrument, Annual Charge, Percentage Other assets Increase (Decrease) in Other Operating Assets Employee Stock Option Share-Based Payment Arrangement, Option [Member] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Investments Net realized (gains) losses on investments Net realized (losses) gains on investments Debt and Equity Securities, Realized Gain (Loss) Total net assets of New Mountain Finance Corporation Equity, Attributable to Parent Drawn revolving credit facilities Drawn Revolving Credit Facilities [Member] Drawn Revolving Credit Facilities Insider Trading Policies and Procedures [Line Items] Schedule of Properties Property, Plant and Equipment [Table Text Block] Common stock, authorized (in shares) Common Stock, Shares Authorized NM YI LLC NM YI, LLC NM YI LLC [Member] NM YI LLC Shares sold (in shares) Stock Issued During Period, Shares, New Issues Ciklum Inc. Ciklum Inc. [Member] Ciklum Inc. New Mountain Net Lease Corporation Investment, Noncontrolling Interest [Member] Investment, Noncontrolling Interest Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Maximum High Maximum [Member] TVG-Edmentum Holdings, LLC, TVG-Edmentum Holdings, LLC, [Member] TVG-Edmentum Holdings, LLC, After Reinvestment Period After Reinvestment Period [Member] After Reinvestment Period Less: Net increase in net assets resulting from operations related to non-controlling interest in New Mountain Net Lease Corporation Net Income (Loss) Attributable to Noncontrolling Interest Borrowings Long-Term Debt, Unclassified [Abstract] New Mountain Capital New Mountain Capital [Member] New Mountain Capital Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Debt Instrument [Line Items] Debt Instrument [Line Items] Incentive fee payable Incentive Fee Payable Investment, Identifier [Axis] Investment, Identifier [Axis] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Investments at fair value Fair value, beginning balance Fair value, ending balance Fair Value Fair value Investment Owned, Fair Value Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Number of investments held Investment Company, Number Of Investments Held Investment Company, Number Of Investments Held Controlled Investment, Affiliated Issuer, Controlled [Member] Indirect administrative expenses Investment Company, Fee Paid Indirectly Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Payable for unsettled securities purchased Payable, Investment, Purchase Entity Shell Company Floating rates Debt Securities, Floating Interest Rate [Member] Debt Securities, Floating Interest Rate Average debt outstanding Long-Term Debt, Average Amount Outstanding Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Statement of Stockholders' Equity [Abstract] Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Line of Credit Facility [Table] Line of Credit Facility [Table] Interest and other financing expenses Interest Expense, Operating Distributions related to non-controlling interest in NMNLC Payments to Noncontrolling Interests Second lien Debt Securities, Second Lien [Member] Debt Securities, Second Lien Receivable from affiliates Other Receivables DT1 Midco Corp DT1 Midco Corp [Member] DT1 Midco Corp Management fee Management Fee Expense Document Fiscal Period Focus NMNLC Credit Facility II NMNLC Credit Facility II [Member] NMNLC Credit Facility II Award Timing Method Award Timing Method [Text Block] Litigation Case [Domain] Litigation Case [Domain] Revenue multiple Measurement Input, Revenue Multiple [Member] Award Type Award Type [Axis] $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 Hedging Relationship [Domain] Hedging Relationship [Domain] PPV Intermediate Holdings LLC PPV Intermediate Holdings LLC [Member] PPV Intermediate Holdings LLC Trading Symbol Entity Address, City or Town 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 Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Income Statement Location [Domain] Statement of Income Location, Balance [Domain] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Entity Emerging Growth Company (Increase) decrease in operating assets: Increase (Decrease) in Operating Assets [Abstract] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Investment Company, Nonconsolidated Subsidiary [Line Items] Investment Company, Nonconsolidated Subsidiary [Line Items] Oranje Holdco, Inc. Oranje Holdco, Inc. [Member] Oranje Holdco, Inc. Deferred financing costs (net of accumulated amortization of $48,017 and $63,971, respectively) Deferred financing costs (net of accumulated amortization) Debt Issuance Costs, Net Gains (losses) recognized Derivative, Gain (Loss) on Derivative, Net Purchase amount per $1,000 principal amount Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Number of shares outstanding (in shares) Common stock, outstanding (in shares) Shares outstanding at end of period (in shares) Common Stock, Shares, Outstanding Administration Agreement Administration Agreement [Member] Administration Agreement Firebird Acquisition Corp, Inc. Firebird Acquisition Corp, Inc. [Member] Firebird Acquisition Corp, Inc. Foreign currency Net change in unrealized appreciation on translation of assets and liabilities in foreign currencies Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Interest Rate Debt Instrument, Interest Rate, Effective Percentage Entity Address, Postal Zip Code Restatement Determination Date Restatement Determination Date Total income tax provision Income Tax Expense (Benefit) Interest rate margin Debt Instrument, Basis Spread on Variable Rate Net (decrease) increase in net assets Stockholders' Equity, Period Increase (Decrease) Other assets Other Assets Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Document Information [Line Items] Cash paid for purchase of investments Payment for Purchase, Investment, Operating Activity Preferred stock, issued (in shares) Preferred Stock, Shares Issued Unsecured Notes Unsecured notes outstanding balance Unsecured Debt Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Net Lease Real Estate Sector [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Hedging Relationship [Axis] Hedging Relationship [Axis] Investments in Majority-owned Subsidiaries Investment, Affiliated Issuer, Controlled, Majority-Owned [Member] Holdings Credit Facility, Broadly Syndicated Loans Holdings Credit Facility, Broadly Syndicated Loans [Member] Holdings Credit Facility, Broadly Syndicated Loans Prime Rate Prime Rate [Member] $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 Stock repurchase program Stockholders' Equity, Policy [Policy Text Block] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Issuance discount percentage Guaranteed Debt, Issuance Discount Guaranteed Debt, Issuance Discount Ambrosia Topco LLC Ambrosia Topco LLC [Member] Ambrosia Topco LLC All Trading Arrangements All Trading Arrangements [Member] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Redemption as a percentage of principal Debt Instrument, Redemption Price, Percentage Coupa Holdings, LLC Coupa Holdings, LLC [Member] Coupa Holdings, LLC Commitments and Contingencies Disclosure [Abstract] Net expenses Investment Company, Expense, after Reduction of Fee Waiver and Reimbursement Net cash flows provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation 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 Term of debt Debt Instrument, Term Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Investments in and Advances to Affiliates, at Fair Value [Roll Forward] CRCI Longhorn Holdings, Inc. CRCI Longhorn Holdings, Inc. [Member] CRCI Longhorn Holdings, Inc. Borrowing capacity, fifth percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Subsequent Event Subsequent Event [Member] Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] PEO Total Compensation Amount PEO Total Compensation Amount Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Debt to equity ratio, maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Packaging Container and Packaging Sector [Member] PIK Investment, Interest Rate, Paid in Kind 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 Estimated fair value, liability Derivative Liability, Subject to Master Netting Arrangement, before Offset Schedule of Investments [Abstract] Equity Components [Axis] Equity Components [Axis] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] 2022 Convertible Notes Convertible Debt [Member] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Market & income approach Valuation, Market And Income Approach [Member] Valuation, Market And Income Approach Number of reportable segments Number of Reportable Segments Lender Cost Of Funds Rate Lender Cost Of Funds Rate [Member] Lender Cost Of Funds Rate Regulation Regulatory Capital Requirements For Investment Companies [Text Block] Regulatory Capital Requirements For Investment Companies Entity Address, State or Province Borrowing capacity, first percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Delayed Draw Term Loan (DDTL) Delayed Draw Term Loan (DDTL) [Member] Non-controlled/Affiliated Investment, Affiliated Issuer, Noncontrolled [Member] Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] Individual: Individual [Axis] Annualized Catch-Up Threshold Investment Management Agreement, Annualized Catch-Up Threshold [Member] Investment Management Agreement, Annualized Catch-Up Threshold Accounting Changes and Error Corrections [Abstract] Distributions paid Payments of Ordinary Dividends, Common Stock Notorious Topco, LLC Notorious Topco, LLC [Member] Notorious Topco, LLC OA Buyer, Inc. OA Buyer, Inc. [Member] OA Buyer, Inc. Borrowing capacity, fourth percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Repayment of long-term debt Repayments of Long-Term Debt Distributions Investment Company, Distribution [Policy Text Block] Entity Interactive Data Current $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 Consumer Services Consumer Services Sector [Member] Consumer Services Sector Healthcare Healthcare Sector [Member] Community Management Holdings MidCo 2, LLC Community Management Holdings MidCo 2, LLC [Member] Community Management Holdings MidCo 2, LLC Unearned interest income Investment Owned, Unearned Interest Income Investment Owned, Unearned Interest Income Investments, cost Cost Investment Owned, Cost Total net derivatives Total net derivatives Derivative Asset (Liability), Net, Subject To Master Netting Arrangement, Before Offset Derivative Asset (Liability), Net, Subject To Master Netting Arrangement, Before Offset 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 Business Services Business Services Sector [Member] Business Services Sector Average weighted shares outstanding for the period (in shares) Weighted Average Number Of Shares Outstanding Weighted Average Number Of Shares Outstanding Mai Capital Management Intermediate LLC Mai Capital Management Intermediate LLC [Member] Mai Capital Management Intermediate LLC 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 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) Safety Borrower Holdings LLC Safety Borrower Holdings LLC [Member] Safety Borrower Holdings LLC Entity Central Index Key PEO Name PEO Name Conversion price (in dollars per share) Debt Instrument, Convertible, Conversion Price Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Pioneer Buyer I, LLC Pioneer Buyer I, LLC [Member] Pioneer Buyer I, LLC Net change in unrealized appreciation (depreciation): Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net [Abstract] Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net Investment Company, Changes in Net Assets [Table] Investment Company, Change in Net Assets [Table] Schedule of Carrying value of Hedged Assets and Liabilities Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Exercise Price Award Exercise Price Entity Filer Category Dividend Income Dividend income Dividend Income, Operating Class of Stock [Domain] Class of Stock [Domain] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Maximum purchase amount Debt Instrument, Repurchase Face Amount, Maximum Debt Instrument, Repurchase Face Amount, Maximum Nonrelated parties Nonrelated Party [Member] Black Elk Trustee Black Elk Trustee [Member] Black Elk Trustee Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] SBA-guaranteed debentures SBA-guaranteed debentures outstanding Guaranteed Debt Guaranteed Debt Entity Address, Address Line One Deferred offering costs Deferred Charges, Policy [Policy Text Block] Investments [Domain] Investments [Domain] Conversion of 2022 Convertible Notes to Shares (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Capital gains based incentive fee payable Incentive Fee Payable, Capital-Gain-Based Base management fee percentage Management and Service Fees, Base Rate Industry Sector [Axis] Industry Sector [Axis] Fair Value as of Grant Date Award Grant Date Fair Value Consumer Products Consumer Products Sector [Member] Consumer Products Sector Extending Lenders Extending Lenders [Member] Extending Lenders Schedule of Information Related to Borrowings Schedule of Long-Term Debt Instruments [Table Text Block] Repurchase of common stock (in dollars per share) Investment Company, Share Repurchase, Per Share Related Party Transaction [Line Items] Related Party Transaction [Line Items] Adjustment for interest on 2022 Convertible Notes and incentive fees, net Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities Document Quarterly Report Gross redemptions Investments in and Advances to Affiliates, at Fair Value, Gross Reductions Idera, Inc. Idera, Inc. [Member] Idera, Inc. Viper Bidco. Inc. Viper Bidco. Inc. [Member] Viper Bidco. Inc. Borrowings Long-Term Debt, Fair Value Schedule of Unobservable Inputs Used in the Fair Value Measurement Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Counterparty Name [Domain] Counterparty Name [Domain] Calabrio, Inc. Calabrio, Inc. [Member] Calabrio, Inc. Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Investment Type [Axis] Investment Type [Axis] Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Maximum secured debt ratio Debt Instrument, Covenant, Secured Debt Ratio, Maximum Debt Instrument, Covenant, Secured Debt Ratio, Maximum Non-PEO NEO Non-PEO NEO [Member] Investment Company, Changes in Net Assets [Line Items] Investment Company, Change in Net Assets [Line Items] Sale of Stock [Axis] Sale of Stock [Axis] All Award Types Award Type [Domain] Symplr Software Intermediate Holdings, Inc. Symplr Software Intermediate Holdings, Inc. [Member] Symplr Software Intermediate Holdings, Inc. Non-controlling interest in New Mountain Net Lease Corporation Equity, Attributable to Noncontrolling Interest Equity Awards Adjustments Equity Awards Adjustments [Member] Non-cash investment income Other Noncash Income (Expense) Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Other income Other Income Other Income Compensation Amount Outstanding Recovery Compensation Amount Line Of Credit, Class B Advance Line Of Credit, Class B Advance [Member] Line Of Credit, Class B Advance Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Ambrosia Holdco Corp. Ambrosia Holdco Corp. [Member] Ambrosia Holdco Corp. HS Purchaser, LLC / Help/Systems Holdings, Inc. HS Purchaser, LLC / Help/Systems Holdings, Inc. [Member] HS Purchaser, LLC / Help/Systems Holdings, Inc. Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Fair Value Estimate of Fair Value Measurement [Member] Investment period Investment Company, Nonconsolidated Subsidiary, Investment Period Investment Company, Nonconsolidated Subsidiary, Investment Period $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 Schedule of Investments [Line Items] Schedule of Investments [Line Items] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Fixed rates Debt Securities, Fixed Interest Rate [Member] Debt Securities, Fixed Interest Rate Investment additional period Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Australia AUSTRALIA Investment income Revenues [Abstract] Variable Rate Period [Axis] Variable Rate Period [Axis] Variable Rate Period SBA-guaranteed debentures Guaranteed Debt [Member] Guaranteed Debt Fair value Property, Plant, and Equipment, Fair Value Disclosure SBIC II New Mountain Finance SBIC II, L.P. [Member] New Mountain Finance SBIC II, L.P. Financial Instruments [Domain] Financial Instruments [Domain] Purchases, including capitalized PIK and revolver fundings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Cumulative Amount of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Ratio to average net assets: Investment Company, Financial Ratios [Abstract] New Permian Holdco, L.L.C. New Permian Holdco, L.L.C. [Member] New Permian Holdco, L.L.C. Minimum Low Minimum [Member] Funded Debt Investments Funded Debt Securities [Member] Funded Debt Securities 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 2022 Convertible Notes Convertible Debt Company Selected Measure Name Company Selected Measure Name Fair Value Measurement [Domain] Fair Value Measurement [Domain] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Portfolio turnover Investment Company, Portfolio Turnover Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Agent fee percentage Line Of Credit Facility, Agent Fee, Percentage Line Of Credit Facility, Agent Fee, Percentage Provision for taxes Provision for taxes Gain (Loss), Investment and Derivative, Operating, Tax Expense (Benefit) 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 Cash paid for purchase of non-controlling interest in New Mountain Net Lease Corporation Payments To Acquire Additional Interest In Subsidiaries, Operating Activity Payments To Acquire Additional Interest In Subsidiaries, Operating Activity Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] 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 Financial Support to Nonconsolidated Legal Entity [Axis] Financial Support to Nonconsolidated Legal Entity [Axis] Total net (decrease) increase in net assets resulting from capital transactions Investment Company, Capital Share Transaction, Increase (Decrease) Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Entity Small Business Offering costs paid Offering expenses Payments of Stock Issuance Costs eResearchTechnology, Inc. eResearchTechnology, Inc. [Member] eResearchTechnology, Inc. $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 Hong Kong HONG KONG Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Daxko Acquisition Corporation Daxko Acquisition Corporation [Member] Daxko Acquisition Corporation Non-usage fee Non-usage fee Line of Credit Facility, Commitment Fee Amount Distribution payable Distribution Payable Borrowings Long-Term Debt [Text Block] Document Transition Report Summary of Investment Holdings [Line Items] Summary of Investment Holdings [Line Items] Schedule of Principal Amounts and Fair Values of the Company’s Borrowings Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table Text Block] Underlying Securities Award Underlying Securities Amount Repayment of SBA-guaranteed debentures Repayment of SBA-guaranteed debentures Repayments of Other Long-Term Debt 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 Equity Component [Domain] Equity Component [Domain] Document Period End Date Accrued capital gains incentive fees Incentive Fee Expense, Capital-Gain-Based United Kingdom UNITED KINGDOM Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Fullsteam Operations LLC Fullsteam Operations LLC [Member] Fullsteam Operations LLC Quarterly Hurdle Rate Investment Management Agreement, Quarterly Hurdle Rate [Member] Investment Management Agreement, Quarterly Hurdle Rate Schedule of Investments [Table] Schedule of Investments [Table] Income Approach Valuation, Income Approach [Member] Investments including delayed draw facilities Investments Including Delayed Draw Facilities [Member] Investments Including Delayed Draw Facilities Eclipse Buyer Inc Eclipse Buyer Inc [Member] Eclipse Buyer Inc Total net assets Beginning of the period Ending of the period Equity, Including Portion Attributable to Noncontrolling Interest New Benevis Topco, LLC New Benevis Topco, LLC [Member] New Benevis Topco, LLC Amortization of debt discount (premium) Amortization of premium Amortization of discount Amortization of Debt Discount (Premium) Accrual for deferred financing costs Debt Issuance Costs Incurred but Not Yet Paid Debt Issuance Costs Incurred but Not Yet Paid Document Type Level III Fair Value, Inputs, Level 3 [Member] Earnings Per Share Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Paid in capital in excess of par Additional Paid in Capital Brave Parent Holdings, Inc. Brave Parent Holdings, Inc. [Member] Brave Parent Holdings, Inc. PetVet Care Centers, LLC PetVet Care Centers, LLC [Member] PetVet Care Centers, LLC Food & Beverage Food and Beverage Sector [Member] Statistical Measurement [Axis] Statistical Measurement [Axis] Investment Company, Nonconsolidated Subsidiary [Domain] Investment Company, Nonconsolidated Subsidiary [Domain] Commitment fee percentage on unused capacity Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Stock repurchase program, authorized amount Share Repurchase Program, Authorized, Amount Higginbotham Insurance Agency, Inc. Higginbotham Insurance Agency, Inc. [Member] Higginbotham Insurance Agency, Inc. Derivative fixed interest rate Derivative, Fixed Interest Rate Nelipak Holding Company Nelipak Holding Company [Member] Nelipak Holding Company Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Investments threshold period past due for nonaccrual status Investment Owned, Nonaccrual, Threshold Period Past Due, Investment Owned, Nonaccrual, Threshold Period Past Due, Borrowing capacity, third percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Schedule of Effect of Hedging Derivative Instruments Derivative Instruments, Gain (Loss) [Table Text Block] Other Other Valuation Technique [Member] Other Valuation Technique Beginning of the period (in shares) End of the period (in shares) Shares, Outstanding Level I Fair Value, Inputs, Level 1 [Member] Credit facility Credit facility outstanding balance Long-Term Line of Credit Schedule of Related Party Transactions, by Related Party [Table] Related Party Transaction [Table] Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Redemption of shares paid-in-kind Investment Company, Redemption of Shares, Paid-In-Kind Investment Company, Redemption of Shares, Paid-In-Kind Foundational Education Group, Inc. Foundational Education Group, Inc. [Member] Foundational Education Group, Inc. Payable to broker Due to Correspondent Brokers Low Voltage Holdings Inc. Low Voltage Holdings Inc. [Member] Low Voltage Holdings Inc. Sale of stock, issuance amount Sale Of Stock, Maximum Issuance, Amount Sale Of Stock, Maximum Issuance, Amount Sale of stock, number of shares issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Treasury stock, common (in shares) Treasury Stock, Common, Shares Total incentive fee Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver Securities Financing Transaction [Table] Security Financing Transaction [Table] 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 Investment Company, Nonconsolidated Subsidiary [Table] Investment Company, Nonconsolidated Subsidiary [Table] Interest income (excluding Payment-in-kind ("PIK") interest income) Interest Income, Operating, Paid in Cash Investment Company, Financial Highlights [Table] Investment Company, Financial Highlights [Table] Income taxes (received) paid Income Taxes Paid, Net Financial Services Financial Services Sector [Member] Bonterra LLC Bonterra LLC [Member] Bonterra LLC Outstanding balance Long-Term Debt, Gross Spread Investment, Basis Spread, Variable Rate Numerator for diluted earnings per share: Net Income (Loss) Attributable to Parent, Diluted Available borrowing capacity Line of Credit Facility, Current Borrowing Capacity Education Education Sector [Member] Education Sector Principal amount, shares (in shares) Investment Owned, Balance, Shares Expiration Date Trading Arrangement Expiration Date Other Commitments [Domain] Other Commitments [Domain] Net investment income (in dollars per share) Investment Company, Investment Income (Loss), Per Share, Basic Other liabilities Increase (Decrease) in Other Operating Liabilities RLG Holdings, LLC RLG Holdings, LLC [Member] RLG Holdings, LLC PIK interest income Interest Income, Operating, Paid in Kind Total Investments [Member] Net realized (losses) gains: Net Realized Gains (Losses) [Abstract] Net Realized Gains (Losses) Next Holdco, LLC Next Holdco, LLC [Member] Next Holdco, LLC Non-cash activity on investments Noncash Activity On Investments Noncash Activity On Investments Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Jersey JERSEY Entity Current Reporting Status Schedule of Investments Investment Holdings, Schedule of Investments [Table Text Block] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Distribution & Logistics Transportation Sector [Member] Total expenses, before waivers/reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Financial Instrument [Axis] Financial Instrument [Axis] Statement of Financial Position [Abstract] Executive Category: Executive Category [Axis] Current Fiscal Year End Date 2022 Convertible Notes 2022 Convertible Notes [Member] 2022 Convertible Notes Conversion of 2022 Convertible Notes to Shares Debt Conversion, Converted Instrument, Amount Realized Capital Gains Investment Management Agreement , Realized Capital Gains [Member] Investment Management Agreement , Realized Capital Gains Professional fees Professional Fees Derivative liability at fair value Derivative Liability Ultimus Group Midco, LLC Ultimus Group Midco, LLC [Member] Ultimus Group Midco, LLC Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Investments excluding drawn revolvers Investments Excluding Drawn Revolvers [Member] Investments Excluding Drawn Revolvers MRI Software LLC MRI Software LLC [Member] MRI Software LLC SBIC III New Mountain Finance SBIC III, L.P. [Member] New Mountain Finance SBIC III, L.P. 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 Upfront fees percentage Guaranteed Debt, Upfront Fees Guaranteed Debt, Upfront Fees Financial Support to Nonconsolidated Legal Entity [Domain] Financial Support to Nonconsolidated Legal Entity [Domain] Structured Finance Obligations Structured Finance Obligations [Member] Structured Finance Obligations $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 Net increase in members' capital Increase (Decrease) In Member's Capital Increase (Decrease) In Member's Capital Energy Energy Sector [Member] Other Commitments [Table] Other Commitments [Table] Net cash flows used in financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Measure: Measure [Axis] NMC Crimson Holdings, Inc. NMC Crimson Holdings, Inc. [Member] NMC Crimson Holdings, Inc. Interest expense recognized on derivative Interest Expense Recognized on Derivative Interest Expense Recognized on Derivative Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Carrying Value Derivative, Fair Value, Net During Reinvestment Period During Reinvestment Period [Member] During Reinvestment Period Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Interest and dividend receivable Dividend and Interest Receivable NMFC Credit Facility NMFC Credit Facility [Member] NMFC Credit Facility CB Buyer, Inc. CB Buyer, Inc. [Member] CB Buyer, Inc. Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Amortization of deferred financing costs Amortization of financing costs Amortization of Debt Issuance Costs TigerConnect, Inc. TigerConnect, Inc. [Member] TigerConnect, Inc. Total expenses Investment Income, Investment Expense Entity Tax Identification Number Incentive fee waived Less: incentive fee waiver Investment Company, Incentive Fee, Contractual Fee Waived Investment Funds (includes investments in joint ventures) Investment Funds, Including Investments In Joint Ventures [Member] Investment Funds, Including Investments In Joint Ventures Common stock, issued (in shares) Common Stock, Shares, Issued Reorganized Careismatic Brands, LLC Reorganized Careismatic Brands, LLC [Member] Reorganized Careismatic Brands, LLC SkyKnight Alpha SkyKnight Income Alpha, LLC [Member] SkyKnight Income Alpha, LLC Payment received for settlement Proceeds from Legal Settlements Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Number of incentive fee components Management And Service Fees, Incentive Rate, Number Of Components Management And Service Fees, Incentive Rate, Number Of Components Security Exchange Name Deferred financing costs, accumulated amortization Accumulated Amortization, Debt Issuance Costs 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 Securities Purchased under Agreements to Resell Drawn revolvers Drawn Revolvers [Member] Drawn Revolvers Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Per share market value (in dollars per share) Investment Company, Market Value, Per Share Interest and other financing expenses Interest Expense, Policy [Policy Text Block] Expenses Operating Expenses [Abstract] Aggregate principal amount converted Debt Conversion, Original Debt, Amount Asset Class [Domain] Asset Class [Domain] Total gains or losses included in earnings: Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Incentive fee payable Increase (Decrease) in Incentive Fee Payable Statement [Line Items] Statement [Line Items] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents Cash and Cash Equivalent Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] 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 Subsequent Event [Line Items] Subsequent Event [Line Items] Deferred tax asset Increase (Decrease) in Deferred Income Taxes Common Stock Common Stock [Member] Capital share activity Investment Company, Capital Share Transactions, Stock Issued [Abstract] Distributions paid per share (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Statistical Measurement [Domain] Statistical Measurement [Domain] Related Party Transaction [Axis] Related Party Transaction [Axis] Number of collateralized agreements Number Of Collateralized Agreements Number Of Collateralized Agreements Asset Class [Axis] Asset Class [Axis] Dividend income Dividend Income, Operating, Paid in Cash Interest rate swaps Interest Rate Swap [Member] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Deferred financing costs Debt, Policy [Policy Text Block] Earnings per share—basic Earnings Per Share, Basic [Abstract] UniTek Global Services, Inc. UniTek Global Services, Inc. [Member] UniTek Global Services, Inc. Foreside Financial Group, LLC Foreside Financial Group, LLC [Member] Foreside Financial Group, LLC CG Group Holdings, LLC CG Group Holdings, LLC [Member] CG Group Holdings, LLC Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Basic earnings per share (in dollars per share) Earnings Per Share, Basic Accumulated Net Realized (Losses) Gains Retained Earnings, Accumulated Net Realized Gain (Loss) [Member] Retained Earnings, Accumulated Net Realized Gain (Loss) Discount rate Measurement Input, Discount Rate [Member] EBITDA multiple Measurement Input, EBITDA Multiple [Member] $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 Measurement Input Type [Domain] Measurement Input Type [Domain] Affiliates Affiliated Entity [Member] Net change in 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) Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Other general and administrative expenses Other General and Administrative Expense Variable Rate Period [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] Average net assets for the period Investment Company, Average Net Assets Number of fee components Management And Service Fees, Number Of Components Management And Service Fees, Number Of Components Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Hedging Designation [Domain] Hedging Designation [Domain] Total net increase (in dollars per share) Investment Company, Investment Income (Loss) from Operations, Per Share Total liabilities and net assets Liabilities and Equity Foreign securities Foreign Currency Transactions and Translations Policy [Policy Text Block] 6.875% Notes due 2029 Six Point Eight Seven Five Percentage Notes Due Two Thousand Twenty Nine [Member] Six Point Eight Seven Five Percentage Notes Due Two Thousand Twenty Nine 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 Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Cash distribution received for purchase of non-controlling interest in New Mountain Net Lease Corporation Proceeds From Noncontrolling Interests, Operating Activity Proceeds From Noncontrolling Interests, Operating Activity Net change in unrealized (depreciation) appreciation of investments Debt and Equity Securities, Unrealized Gain (Loss) [Member] Debt and Equity Securities, Unrealized Gain (Loss) Accounting Policies [Abstract] 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 8.25% Notes due 2028 Eight Point Two Five Percentage Notes Due Two Thousand Twenty Eight [Member] Eight Point Two Five Percentage Notes Due Two Thousand Twenty Eight Concentration Risk Type [Domain] Concentration Risk Type [Domain] Less: management and incentive fees waived (See Note 5) Investment Company, Contractual Fee Waived PDI TA Holdings, Inc. PDI TA Holdings, Inc. [Member] PDI TA Holdings, Inc. Investments at fair value Investments, At Fair Value [Member] Investments, At Fair Value Total expenses, net of waivers/reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Treasury Stock at Cost Treasury Stock, Common [Member] Pay vs Performance Disclosure [Line Items] 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 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 Liabilities Liabilities [Abstract] Summary of Investment Holdings [Table] Investment Holdings [Table] Weighted average interest rate Long-Term Debt, Weighted Average Interest Rate, over Time 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] Auctane Inc. (fka Stamps.com Inc.) Auctane Inc. (fka Stamps.com Inc.) [Member] Auctane Inc. (fka Stamps.com Inc.) GS Acquisitionco, Inc GS Acquisitionco, Inc [Member] GS Acquisitionco, Inc SkyKnight II SkyKnight Income II, LLC [Member] SkyKnight Income II, LLC Interest and dividend receivable Increase (Decrease) in Interest and Dividends Receivable Statement of Cash Flows [Abstract] Company Selected Measure Amount Company Selected Measure Amount Specialtycare, Inc. Specialtycare, Inc. [Member] Specialtycare, Inc. Measurement Input Type [Axis] Measurement Input Type [Axis] Investment, Issuer Affiliation [Axis] Investment, Issuer Affiliation [Axis] Name Measure Name Industry Sector [Domain] Industry Sector [Domain] Trinity Air Consultants Holdings Corporation Trinity Air Consultants Holdings Corporation [Member] Trinity Air Consultants Holdings Corporation Deferred tax liability Deferred Income Tax Liabilities, Net Permian Trust Permian Trust [Member] Permian Trust Unsecured Notes Unsecured Debt [Member] Net borrowings Long-Term Debt Legal Entity [Axis] Total assets Assets Payable to broker Increase (Decrease) Due To Correspondent Brokers Increase (Decrease) Due To Correspondent Brokers Non-cash dividend income Dividend Income, Operating, Paid in Kind DOCS, MSO, LLC DOCS, MSO, LLC [Member] DOCS, MSO, LLC Geographical [Domain] Geographical [Domain] Income taxes Income Tax, Policy [Policy Text Block] Cash flows from operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Receivable from unsettled securities sold Receivable, Investment, Sale Concentration Risk Type [Axis] Concentration Risk Type [Axis] GraphPAD Software, LLC GraphPAD Software, LLC [Member] GraphPAD Software, LLC Conversion premium Debt Instrument, Convertible Conversion Premium Debt Instrument, Convertible Conversion Premium Warrants Warrant [Member] Distributions declared per share (in dollars per share) Common Stock, Dividends, Per Share, Declared Entity Common Stock, Shares Outstanding Principal amount, par value Principal Amount or Par Value Principal amount Investment Owned, Balance, Principal Amount RailPros Parent, LLC RailPros Parent, LLC [Member] RailPros Parent, LLC Management fee waived Less: management fee waiver Investment Company, Management Fee, Contractual Fee Waived Accrual for offering costs Stock Issuance Costs Incurred but Not Yet Paid Stock Issuance Costs Incurred but Not Yet Paid Level II Fair Value, Inputs, Level 2 [Member] 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 Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Collateralized agreements or repurchase financings Repurchase Agreements, Collateral, Policy [Policy Text Block] American Achievement Corporation (aka AAC Holding Corp.) American Achievement Corporation (aka AAC Holding Corp.) [Member] American Achievement Corporation (aka AAC Holding Corp.) Schedule of Current Income Tax Expense (Benefit), Deferred Income Tax Provision and Total Income Tax Provision Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Legends Hospitality Holding Company, LLC Legends Hospitality Holding Company, LLC [Member] Legends Hospitality Holding Company, LLC Document Information [Table] PEO PEO [Member] Transfers into Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Intercompany Revolver Intercompany Revolver [Member] Intercompany Revolver United States UNITED STATES Cash and cash equivalents at the beginning of the period Cash and cash equivalents at the end of the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Income Statement [Abstract] Derivative asset at fair value Derivative Asset Fees waived by the administrator Investment Company, Voluntary Fee Waived Sale of Stock [Domain] Sale of Stock [Domain] Total investments Investments, Fair Value Disclosure Interest payable Increase (Decrease) in Interest Payable, Net Total return based on net asset value Investment Company, Total Return Sierra Hamilton Holdings Corporation Sierra Hamilton Holdings Corporation [Member] Sierra Hamilton Holdings Corporation Pre-Incentive Fee Net Investment Income Investment Management Agreement, Pre-Incentive Fee Net Investment Income [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Net investment income before income taxes Net investment income Net Investment Income New Mountain Finance Corporation Total Net Assets of NMFC Parent [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 Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Notional Amount Notional amount Derivative, Notional Amount Organization, Consolidation and Presentation of Financial Statements [Abstract] Distributions declared to stockholders from net investment income Distributions declared Investment Company, Dividend Distribution City Area Code Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Specialty Chemicals & Materials Chemicals Sector [Member] Regulatory capital Small Business Association Regulation, Regulatory Capital, Actual Small Business Association Regulation, Regulatory Capital, Actual 6.875% Notes due 2029 And 6.200% Notes due 2027 Six Point Eight Seven Five Percentage Notes Due Two Thousand Twenty Nine And Six Point Two Percentage Notes Due Two Thousand Twenty Seven [Member] Six Point Eight Seven Five Percentage Notes Due Two Thousand Twenty Nine And Six Point Two Percentage Notes Due Two Thousand Twenty Seven Proceeds from credit facility Proceeds from Long-Term Lines of Credit Groundworks, LLC Groundworks, LLC [Member] Groundworks, LLC Park Place Technologies, LLC Park Place Technologies, LLC [Member] Park Place Technologies, LLC Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Fair Value Hedging Fair Value Hedging [Member] Related Party Transactions [Abstract] Incentive fee Incentive Fee Expense Assets under management Assets under Management, Carrying Amount Incentive fee, excluding accrued capital gains incentive fees Incentive Fee Expense, Income-Based Other liabilities Other liabilities Due to affiliates Other Liabilities CFS Management, LLC CFS Management, LLC [Member] CFS Management, LLC Associations, Inc. Associations, Inc. [Member] Associations, Inc. Transfers out of Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 DCA Investment Holding, LLC DCA Investment Holding, LLC [Member] DCA Investment Holding, LLC Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] Supplemental disclosure of cash flow information Supplemental Cash Flow Information [Abstract] Other Performance Measure, Amount Other Performance Measure, Amount Software Software Sector [Member] Software Sector Deferred financing costs paid Payments of Debt Issuance Costs Net investment income Investment Company, Investment Income (Loss) Ratio Fair value Fair Value Measurement, Policy [Policy Text Block] Wealth Enhancement Group, LLC Wealth Enhancement Group, LLC [Member] Wealth Enhancement Group, LLC Related Party [Domain] Related and Nonrelated Parties [Domain] 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 Type Investment Type Concentration Risk [Member] Investment Type Concentration Risk Net increase (decrease) in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Holdings Credit Facility Holdings Credit Facility [Member] Holdings Credit Facility Unearned income Investment Owned, Unearned Income Investment Owned, Unearned Income Derivative Instrument [Axis] Derivative Instrument [Axis] Cash and other assets Cash And Other Assets Cash And Other Assets Variable Rate Component [Domain] Variable Rate Component [Domain] Variable Rate Component [Domain] Entity [Domain] Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Cash interest paid Interest Paid, Excluding Capitalized Interest, Operating Activity Document Fiscal Year Focus Basis of accounting Basis of Accounting, Policy [Policy Text Block] Principal Amount Reported Value Measurement [Member] NC Topco, LLC NC Topco, LLC [Member] NC Topco, LLC 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 Equity [Abstract] Derivative instruments and hedging activities Derivatives, Policy [Policy Text Block] Segment Reporting Segment Reporting Disclosure [Text Block] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] 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] Baker Tilly Advisory Group, LP Baker Tilly Advisory Group, LP [Member] Baker Tilly Advisory Group, LP Other Commitments [Line Items] Other Commitments [Line Items] Holdings Credit Facility, Excluding Broadly Syndicated Loans Holdings Credit Facility, Excluding Broadly Syndicated Loans [Member] Holdings Credit Facility, Excluding Broadly Syndicated Loans Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Weighted average Weighted Average Weighted Average [Member] Convey Health Solutions, Inc. Convey Health Solutions, Inc. [Member] Convey Health Solutions, Inc. Net change in unrealized appreciation in NMNLC Noncontrolling Interest, Net Change In Unrealized Appreciation Noncontrolling Interest, Net Change In Unrealized Appreciation AmeriVet Partners Management, Inc. AmeriVet Partners Management, Inc. [Member] AmeriVet Partners Management, Inc. Conversion rate Debt Instrument, Convertible, Conversion Ratio TVG-Edmentum Holdings, LLC / Edmentum Ultimate Holdings, LLC TVG-Edmentum Holdings, LLC / Edmentum Ultimate Holdings, LLC [Member] TVG-Edmentum Holdings, LLC / Edmentum Ultimate Holdings, LLC Sale of stock, consideration received Sale of stock, proceeds received Sale of Stock, Consideration Received on Transaction Total management fee Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Adjustment for dilutive effect of convertible notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities 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 Receivable from affiliates Increase (Decrease) Due from Affiliates Revolving Credit Facility Revolving Credit Facility [Member] Arrangement Duration Trading Arrangement Duration Investment, Issuer Name [Domain] Investment, Issuer Name [Domain] Power Grid Holdings, Inc. Power Grid Holdings, Inc. [Member] Power Grid Holdings, Inc. Local Phone Number Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Payable for unsettled securities purchased Increase (Decrease) in Payable for Investment Purchased Assets Assets [Abstract] Proceeds from Unsecured Notes Proceeds from Issuance of Unsecured Debt Credit Facility [Axis] Credit Facility [Axis] Gross additions Investments in and Advances to Affiliates, at Fair Value, Gross Additions Underlying Security Market Price Change Underlying Security Market Price Change, Percent Other Commitments [Axis] Other Commitments [Axis] Debt Instrument [Axis] Fair Value Fair Value Disclosures [Text Block] Quarterly Catch-Up Threshold Investment Management Agreement, Quarterly Catch-Up Threshold [Member] Investment Management Agreement, Quarterly Catch-Up Threshold Credit Facility [Domain] Credit Facility [Domain] Investments Net change in unrealized depreciation (appreciation) of investments and New Mountain Net Lease Corporation Net change in unrealized (depreciation) appreciation of investments Debt and Equity Securities, Unrealized Gain (Loss) Net assets Equity, Attributable to Parent [Abstract] Percent of Net Assets Investment Owned, Net Assets, Percentage Repurchase of shares under Old Repurchase Program Common stock repurchased Repurchases of common stock under Old Repurchase Program Treasury Stock, Value, Acquired, Cost Method Stated interest rate Debt Instrument, Interest Rate, Stated Percentage 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 Management fee payable Management Fee Payable Estimated fair value, asset Derivative Asset, Subject to Master Netting Arrangement, before Offset Investment Company [Abstract] Financial Highlights Investment Company, Financial Highlights [Text Block] Entity Registrant Name Investment Company, Financial Highlights [Line Items] Investment Company, Financial Highlights [Line Items] Securities Financing Transaction [Line Items] Securities Financing Transaction [Line Items] Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Investment Management Agreement Investment Management Agreement [Member] Investment Management Agreement YLG Holdings, Inc. YLG Holdings, Inc. [Member] YLG Holdings, Inc. National HME, Inc./NHME Holdings Corp. National HME, Inc. And NHME Holdings Corp. [Member] National HME, Inc. And NHME Holdings Corp. Offering costs Offering costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Capital transactions Investment Company, Capital Share Transaction, Increase (Decrease) [Abstract] Unsecured Management Company Revolver Unsecured Management Company Revolver [Member] Unsecured Management Company Revolver Number of operating segments Number of Operating Segments Amortization of purchase discount Accretion (Amortization) of Discounts and Premiums, Investments Unfunded commitments on bridge facilities Unfunded Debt Securities, Bridge Facilities [Member] Unfunded Debt Securities, Bridge Facilities Investment, percentage of interest entitled to receive Investment, Percentage Of Interest Entitled To Receive Investment, Percentage Of Interest Entitled To Receive Repayment of 2022 Convertible Notes Repayment of 2022 Convertible Notes Repayments of Convertible Debt Increase (decrease) in net assets resulting from operations: Net Income (Loss), Including Portion Attributable to Noncontrolling Interest [Abstract] Related Party [Axis] Related and Nonrelated Parties [Axis] Derivative liability, fair value, offset against collateral, net Derivative Liability, Subject to Master Netting Arrangement, after Offset and Deduction Non-cash operating activities: Non Cash Operating And Investing Activities [Abstract] Non Cash Operating And Investing Activities Repayments of credit facility Repayments of Long-Term Lines of Credit Restatement Determination Date: Restatement Determination Date [Axis] NM NL Holdings, L.P. NM NL Holdings, L.P. [Member] NM NL Holdings, L.P. Members' capital Members' Equity Adelaide Borrower, LLC Adelaide Borrower, LLC [Member] Adelaide Borrower, LLC KPSKY Acquisition Inc. KPSKY Acquisition Inc. [Member] KPSKY Acquisition Inc. Net realized and unrealized losses Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax USRP Holdings, Inc. USRP Holdings, Inc. [Member] USRP Holdings, Inc. Interest expense Interest expense Interest Expense, Debt Name Trading Arrangement, Individual Name Unfunded commitments principal amount Unfunded commitments principal amount Investment Company, Financial Support to Investee Contractually Required, Not Provided, Amount Other Stock Issued During Period, Value, Other Sale of non-controlling interest in NMNLC Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Face amount Debenture Amount Debt Instrument, Face Amount Repurchase amount Investment Owned, Balance, Repurchase Amount Investment Owned, Balance, Repurchase Amount Unfunded Debt Investments Unfunded Debt Securities [Member] Unfunded Debt Securities Investment, Issuer Affiliation [Domain] Investment, Issuer Affiliation [Domain] Name Awards Close in Time to MNPI Disclosures, Individual Name Eclipse Topco Holdings, Inc Eclipse Topco Holdings, Inc. (fka Transcendia Holdings, Inc.) [Member] Eclipse Topco Holdings, Inc. (fka Transcendia Holdings, Inc.) 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. Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Other commitments Other Commitment Conversion of 2022 Convertible Notes to Shares Stock Issued During Period, Value, Conversion of Convertible Securities Flash Charm Inc. (fka Idera, Inc.) Flash Charm Inc. (fka Idera, Inc.) [Member] Flash Charm Inc. (fka Idera, Inc.) Funded Investments Funded Securities [Member] Funded Securities Litigation Case [Axis] Litigation Case [Axis] OEConnection LLC OEConnection LLC [Member] OEConnection LLC Income Statement Location [Axis] Statement of Income Location, Balance [Axis] Damages sought Loss Contingency, Damages Sought, Value Maximum amount of borrowings available Line of Credit Facility, Remaining Borrowing Capacity 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 Net change in unrealized (depreciation) appreciation of investments, foreign currency and New Mountain Net Lease Corporation Net Change In Unrealized Appreciation (Depreciation) Unrealized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Asset Derivative Asset [Abstract] Earnings per share Earnings Per Share, Policy [Policy Text Block] Revenue recognition Revenue [Policy Text Block] Derivative liability, subject to a master netting arrangement Derivative Liability, Subject to Master Netting Arrangement, Asset Offset Preferred stock, par value $0.01 per share, 2,000,000 shares authorized, none issued Preferred Stock, Value, Issued Interest payable Interest Payable Proceeds from sale of investment Proceeds from Disposition of Investment, Operating Activity Subordinated Subordinated Debt [Member] All Executive Categories All Executive Categories [Member] First lien Debt Securities, First Lien [Member] Debt Securities, First Lien Common stock, par value $0.01 per share, 200,000,000 shares authorized, 107,851,929 and 107,851,415 shares issued, respectively, and 104,189,336 and 107,851,415 shares outstanding, respectively Common Stock, Value, Issued Schedule of Information Related to Convertible Notes Convertible Debt [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Infogain Corporation Infogain Corporation [Member] Infogain Corporation Kene Acquisition, Inc. Kene Acquisition, Inc. [Member] Kene Acquisition, Inc. Commitments and contingencies (See Note 9) Commitments and Contingencies (Decrease) increase in operating liabilities: Increase (Decrease) in Operating Liabilities [Abstract] Line Of Credit, Class A Advance Line Of Credit, Class A Advance [Member] Line Of Credit, Class A Advance Measurement input Investment Company, Investment Owned, Measurement Input Non-Controlling Interest in NMNLC Noncontrolling Interest [Member] SBIC I New Mountain Finance SBIC, L.P. [Member] New Mountain Finance SBIC, L.P. Unfunded commitments on delayed draws or 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 Recent Accounting Standards Updates Accounting Standards Update and Change in Accounting Principle [Text Block] Offering costs Adjustments to Additional Paid in Capital, Stock Offering Costs Adjustments to Additional Paid in Capital, Stock Offering Costs Unfunded Investments Unfunded Securities [Member] Unfunded Securities Shares redeemed for a promissory note (in shares) Investment Owned, Shares Redeemed Investment Owned, Shares Redeemed PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount 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 Earnings per share—diluted Earnings Per Share, Diluted [Abstract] Hedging Designation [Axis] Hedging Designation [Axis] Net realized gains (loss) 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 NM GP Holdco, LLC NM GP Holdco, LLC [Member] NM GP Holdco, LLC Derivative Contract [Domain] Derivative Contract [Domain] Net proceeds from shares sold Net proceeds from offerings of common stock Proceeds from Issuance of Common Stock All Individuals All Individuals [Member] Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Fair Value Disclosures [Abstract] Schedule of Summarized Financial Information for Nonconsolidated Entities Investment Company, Nonconsolidated Subsidiary, Summarized Financial Information [Table Text Block] Name Forgone Recovery, Individual Name Distributions declared to stockholders from net investment income (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Net change due to hedging activity Unrealized Gain (Loss) on Derivatives Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Affiliate of Investment Adviser Affiliate Of Investment Adviser [Member] Affiliate Of Investment Adviser 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. 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.) Schedule of Notional Amounts and Fair Values of the Company's Derivative Instruments Schedule of Derivative Instruments [Table Text Block] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Geographical [Axis] Geographical [Axis] Shares issued in debt conversion (in shares) Debt Conversion, Converted Instrument, Shares Issued Liability Derivative Liability [Abstract] 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) Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] 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] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Interest Income Interest income Interest Income, Operating Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Title of 12(b) Security Earnings Per Share [Abstract] Net investment income Net investment income Investment Income, Operating, after Expense and Tax 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 Net expense recognized on fair value hedge Gain (Loss) on Fair Value Hedges Recognized in Earnings Administrative expenses Administrative Fees Expense Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Shares repurchased under Old Repurchase Program (in shares) Shares repurchased under repurchase program (in shares) Repurchases of common stock under Old Repurchase Program (in shares) Treasury Stock, Shares, Acquired Income tax expense Investment Income, Operating, Tax Expense (Benefit) NM APP CANADA CORP NM APP Canada Corp [Member] NM APP Canada Corp Award Timing Disclosures [Line Items] Net Assets Equity [Text Block] Effective interest rate Debt Instrument, Interest Rate During Period 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) Attributable to Parent Subsequent Event [Table] Subsequent Event [Table] Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Repurchase of shares under Old Repurchase Program Payments for Repurchase of Common Stock Adoption Date Trading Arrangement Adoption Date Base Rate Base Rate [Member] Annualized Hurdle Rate Investment Management Agreement, Annualized Hurdle Rate [Member] Investment Management Agreement, Annualized Hurdle Rate Incentive fee percentage Management and Service Fees, Incentive Rate Total coupon/interest rate Interest rate Interest rate Investment Interest Rate Non-cash financing activities: Noncash Financing Items [Abstract] Noncash Financing Items Line of Credit Line of Credit [Member] Dividend reinvestment plan, share price percentage, threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Accumulated undistributed earnings Retained Earnings (Accumulated Deficit) Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Accretive effect of common stock issuances per share (in dollars per share) Investment Company, Accretive Effect Of Common Stock Issuances Per Share Investment Company, Accretive Effect Of Common Stock Issuances Per Share Class of Stock [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Statement [Table] Statement [Table] ATM Offering At the Market Offering [Member] At the Market Offering Deferred income tax provision Deferred Income Tax Expense (Benefit) Effect of foreign exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Including Discontinued Operation Cash flows from financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) [Member] Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) Cardinal Parent, Inc. Cardinal Parent, Inc. [Member] Cardinal Parent, Inc. Variable Rate [Domain] Variable Rate [Domain] Formation and Business Purpose Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Schedule of Changes in Fair Value of Level III Portfolio Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Repayment of Unsecured Notes Repayments of Unsecured Debt Management fee payable Increase (Decrease) in Management Fee Payable All Adjustments to Compensation All Adjustments to Compensation [Member] Amendment Flag Sale of stock, price per share (in dollars per share) Sale of Stock, Price Per Share IG Investments Holdings, LLC IG Investments Holdings, LLC [Member] IG Investments Holdings, LLC Treasury stock at cost, 3,662,593 and 0 shares held, respectively Treasury Stock, Common, Value Termination Date Trading Arrangement Termination Date Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Debt instrument, variable interest rate, type [Extensible Enumeration] Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration] Net realized losses on translation of assets and liabilities in foreign currencies Net realized losses on translation of assets and liabilities in foreign currencies Realized Gain (Loss), Foreign Currency Transaction, before Tax New Benevis Holdco, Inc. New Benevis Holdco, Inc. [Member] New Benevis Holdco, Inc. Shares sold (in shares) Investment Owned, Shares Sold Investment Owned, Shares Sold Schedule of Portfolio Investments by Levels in the Fair Value Hierarchy Fair Value, Assets Measured on Recurring Basis [Table Text Block] Weighted average shares of common stock outstanding - basic (in shares) Denominator for basic weighted average share (in shares) Weighted Average Number of Shares Outstanding, Basic Investment Company, Financial Highlights [Roll Forward] Investment Company, Financial Highlights [Roll Forward] Schedule of Basic and Diluted Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Investment, Issuer Name [Axis] Investment, Issuer Name [Axis] Related Party Transaction [Domain] Related Party Transaction [Domain] Subsequent Events [Abstract] Segment Reporting [Abstract] Schedule of Net Assets Investment Company, Change in Net Assets [Table Text Block] Debt Disclosure [Abstract] Tennessee Bidco Limited Tennessee Bidco Limited [Member] Tennessee Bidco Limited Fortis Solutions Group, LLC Fortis Solutions Group, LLC [Member] Fortis Solutions Group, LLC Galway Borrower LLC Galway Borrower LLC [Member] Galway Borrower LLC 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] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Model N, Inc. Model N, Inc. [Member] Model N, Inc. Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Use of estimates Use of Estimates, Policy [Policy Text Block] Settlement amount awarded to other party Litigation Settlement, Amount Awarded to Other Party Derivative asset, subject to a master netting arrangement Derivative Asset, Subject to Master Netting Arrangement, Liability Offset Subsequent Events Subsequent Events [Text Block] Insider Trading Arrangements [Line Items] 2021A Unsecured Notes Unsecured Notes Effective January 2021 [Member] Unsecured Notes Effective January 2021 Equity Equity Securities [Member] Syndigo LLC Syndigo LLC [Member] Syndigo LLC iCIMS, Inc. iCIMS, Inc. [Member] iCIMS, Inc. Total liabilities Liabilities New Mountain Net Lease Corporation NMNLC New Mountain Net Lease Corporation [Member] New Mountain Net Lease Corporation Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Derivative floating interest rate Derivative, Variable Interest Rate 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.) And Eagle Infrastructure Super HoldCo, LLC [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) And Eagle Infrastructure Super HoldCo, LLC Purchase amount Debt Instrument, Repurchased Face Amount Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Equity and other Other Security Investments [Member] Net proceeds from shares sold Stock Issued During Period, Value, New Issues Variable Rate [Axis] Variable Rate [Axis] Counterparty Name [Axis] Counterparty Name [Axis] Total investment income Gross Investment Income, Operating Net (decrease) increase in shares outstanding (in shares) Stock Issued During Period, Shares, Period Increase (Decrease) Diligent Corporation Diligent Corporation [Member] Diligent Corporation Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Secured Overnight Financing Rate (SOFR) [Member] Federal Home Loan Bank Of Chicago's Fixed Regular Advance Rate ("Bank Advance Rate") [Member] Federal Home Loan Bank Of Chicago's Fixed Regular Advance Rate ("Bank Advance Rate")