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Financial Highlights - Summary (Details)
$ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2023
USD ($)
$ / shares
shares
Dec. 31, 2022
USD ($)
$ / shares
shares
Dec. 31, 2021
USD ($)
$ / shares
shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2023
GBP (£)
shares
Dec. 31, 2023
EUR (€)
shares
Nov. 13, 2023
Dec. 31, 2022
GBP (£)
shares
Dec. 31, 2022
EUR (€)
shares
Dec. 31, 2021
GBP (£)
shares
Dec. 31, 2021
EUR (€)
shares
Sep. 25, 2018
Investment Company, Financial Highlights [Roll Forward]                          
Net asset value, beginning balance (in dollars per share) | $ / shares $ 13.02 $ 13.49 $ 12.62 $ 13.26 $ 13.22                
Net investment income (in dollars per share) | $ / shares 1.57 1.18 1.21 1.20 1.37                
Net realized and unrealized (losses) gains (in dollars per share) | $ / shares (0.23) (0.43) 0.86 (0.60) 0.03                
Total net increase (in dollars per share) | $ / shares 1.34 0.75 2.07 0.60 1.40                
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares (1.49) (1.22) (1.20) (1.24) (1.36)                
Net asset value, ending balance (in dollars per share) | $ / shares 12.87 13.02 13.49 12.62 13.26                
Per share market value (in dollars per share) | $ / shares $ 12.72 $ 12.37 $ 13.70 $ 11.36 $ 13.74                
Total return based on market value 15.48% (0.56%) 31.91% (5.24%) 20.45%                
Total return based on net asset value 10.64% 5.71% 16.97% 5.52% 10.90%                
Shares outstanding at end of period (in shares) | shares 102,558,859 100,937,026 97,907,441 96,827,342 96,827,342 102,558,859 102,558,859   100,937,026 100,937,026 97,907,441 97,907,441  
Average weighted shares outstanding for the period (in shares) | shares 101,118,302 100,202,847 96,952,959 96,827,342 85,209,378                
Average net assets for the period $ 1,323,778 $ 1,344,266 $ 1,261,338 $ 1,168,043 $ 1,154,615                
Ratio to average net assets:                          
Net investment income 12.00% 8.82% 9.32% 10.05% 10.15%                
Total expenses, before waivers/reimbursements 16.54% 13.35% 13.11% 14.56% 14.87%                
Total expenses, net of waivers/reimbursements 16.23% 13.01% 12.05% 13.39% 13.80%                
Asset coverage ratio 187.54% 177.42% 181.21% 180.68% 173.98% 187.54% 187.54%   177.42% 177.42% 181.21% 181.21%  
Portfolio turnover 8.87% 18.01% 35.33% 15.43% 11.58%                
Effect of common stock issuance (in dollars per share) | $ / shares $ 0.00 $ 0.01 $ (0.01) $ 0.00 $ 0.08                
Convertible Notes                          
Ratio to average net assets:                          
Average debt outstanding $ 320,494 $ 228,806 $ 201,250 $ 201,250 $ 234,332                
SBA-guaranteed debentures                          
Ratio to average net assets:                          
Average debt outstanding 300,000 300,000 300,000 285,852 179,408                
Unsecured Notes                          
Ratio to average net assets:                          
Average debt outstanding 438,473 526,829 516,611 453,250 414,949                
Holdings Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance 515,063 618,963 545,263                    
Holdings Credit Facility | Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Average debt outstanding 577,759 581,367 478,016 526,645 598,129                
NMFC Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance 36,813 40,359                      
NMFC Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance     127,192     £ 22,900 € 700   £ 22,900 € 700 £ 16,400 € 0  
NMFC Credit Facility | Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Average debt outstanding 103,244 133,053 132,685 155,497 105,533                
DB Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance 186,400 186,400                      
DB Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance     244,000                    
DB Credit Facility | Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Average debt outstanding 186,422 209,898 209,307 233,649 113,967                
NMNLC Credit Facility | Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Average debt outstanding 0 0 0 0 1,471                
NMNLC Credit Facility II                          
Ratio to average net assets:                          
Credit facility outstanding balance 2,853 3,785                      
NMNLC Credit Facility II | Revolving Credit Facility                          
Ratio to average net assets:                          
Credit facility outstanding balance 2,853 3,785 15,200                    
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility                          
Ratio to average net assets:                          
Average debt outstanding $ 2,506 $ 7,195 $ 3,501 $ 0 $ 0                
8.25% Notes due 2028 | Unsecured Notes                          
Ratio to average net assets:                          
Stated interest rate               8.25%         8.25%