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Summary of Significant Accounting Policies - Narrative (Details)
 3 Months Ended 9 Months Ended 11 Months Ended
Jul. 01, 2024
USD ($)
$ / shares
shares
Mar. 30, 2020
USD ($)
$ / shares
shares
Sep. 30, 2024
USD ($)
agreement
Sep. 30, 2023
USD ($)
Sep. 30, 2024
USD ($)
agreement
Sep. 30, 2023
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2023
numberOfAgreement
Dec. 31, 2023
agreement
Feb. 04, 2016
USD ($)
Securities Financing Transaction [Line Items]                      
Number of collateralized agreements     1   1       1 1  
Securities purchased under collateralized agreements to resell, cost     $ 30,000,000   $ 30,000,000     $ 30,000,000      
Securities purchased under agreements to resell     13,500,000   13,500,000     16,500,000      
Cash equivalents     0   0     0      
PIK interest income     9,050,000 $ 7,917,000 27,735,000 $ 24,807,000          
Non-cash dividend income     $ 8,319,000 6,988,000 $ 23,484,000 20,225,000          
Investments threshold period past due for nonaccrual status     30 days   30 days            
Deferred tax liability     $ 1,100,000   $ 1,100,000     0      
Deferred tax asset     $ 0   $ 0     $ 594,000      
Dividend reinvestment plan, share price percentage, threshold     110.00%   110.00%            
Investments in Majority-owned Subsidiaries                      
Securities Financing Transaction [Line Items]                      
Shares redeemed for a promissory note (in shares) | shares   105,030                  
Redemption of shares paid-in-kind   $ 11,315,000                  
Interest rate   7.00%                  
Common Stock                      
Securities Financing Transaction [Line Items]                      
Stock repurchase program, authorized amount                     $ 50,000,000
Common stock repurchased     $ 0 $ 0 $ 0 $ 0 $ 2,948,000        
Common Stock | New Mountain Net Lease Corporation                      
Securities Financing Transaction [Line Items]                      
Sale of stock, number of shares issued (in shares) | shares   105,030                  
Sale of stock, price per share (in dollars per share) | $ / shares   $ 107.73                  
Sale of stock, consideration received   $ 11,315,000                  
Common Stock | New Mountain Net Lease Corporation | Investments in Majority-owned Subsidiaries                      
Securities Financing Transaction [Line Items]                      
Sale of stock, price per share (in dollars per share) | $ / shares $ 73.39                    
Sale of stock, consideration received $ 4,666,000                    
Shares sold (in shares) | shares 63,575                    
Common Stock | Affiliate Of Investment Adviser | New Mountain Net Lease Corporation                      
Securities Financing Transaction [Line Items]                      
Sale of stock, number of shares issued (in shares) | shares 63,575                    
Sale of stock, price per share (in dollars per share) | $ / shares $ 73.39                    
Sale of stock, consideration received $ 4,666,000