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Financial Highlights - Senior Securities (Details)
$ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2022
USD ($)
$ / shares
Dec. 31, 2021
USD ($)
$ / shares
Dec. 31, 2020
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Dec. 31, 2014
USD ($)
$ / shares
Dec. 31, 2022
GBP (£)
Dec. 31, 2022
EUR (€)
Dec. 31, 2021
GBP (£)
Sep. 25, 2018
Debt Instrument [Line Items]                          
SBA-guaranteed debentures outstanding | $ $ 300,000 $ 300,000 $ 300,000 $ 225,000 $ 165,000 $ 150,000 $ 121,700 $ 117,700 $ 37,500        
SBA-guaranteed debentures, coverage per unit (in dollars per share) | $ / shares $ 1,658 $ 1,686 $ 1,673 $ 1,655 $ 1,718 $ 2,169 $ 2,320 $ 2,128 $ 2,196        
5.75% Unsecured Notes | Unsecured Notes                          
Debt Instrument [Line Items]                          
Stated interest rate                         5.75%
NMFC Credit Facility | Revolving Credit Facility                          
Debt Instrument [Line Items]                          
Credit facility outstanding balance $ 40,359 $ 127,192 $ 165,500             £ 22,900 € 700 £ 16,400