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Fair Value - Principal Amounts and Fair Values of Borrowings (Details) € in Thousands, £ in Thousands, $ in Thousands | Sep. 30, 2023 USD ($) | Sep. 30, 2023 GBP (£) | Sep. 30, 2023 EUR (€) | Dec. 31, 2022 USD ($) | Dec. 31, 2022 GBP (£) | Dec. 31, 2022 EUR (€)
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Principal Amount |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 1,902,436 |  |  | $ 1,997,823 |  |
Principal Amount | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 391,500 |  |  | 531,500 |  |
Principal Amount | Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 260,000 |  |  | 316,816 |  |
Principal Amount | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 300,000 |  |  | 300,000 |  |
Principal Amount | Revolving Credit Facility | Holdings Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 569,563 |  |  | 618,963 |  |
Principal Amount | Revolving Credit Facility | NMFC Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 192,115 | £ 22,850 | € 700 | 40,359 | £ 22,850 | € 700
Principal Amount | Revolving Credit Facility | DB Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 186,400 |  |  | 186,400 |  |
Principal Amount | Revolving Credit Facility | NMNLC Credit Facility II | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 2,858 |  |  | 3,785 |  |
Fair Value |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 1,818,190 |  |  | 1,899,243 |  |
Fair Value | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 363,692 |  |  | 499,551 |  |
Fair Value | Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 264,706 |  |  | 317,071 |  |
Fair Value | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 250,087 |  |  | 250,442 |  |
Fair Value | Revolving Credit Facility | Holdings Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 560,767 |  |  | 604,971 |  |
Fair Value | Revolving Credit Facility | NMFC Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 190,537 | £ 22,662 | € 694 | 39,699 | £ 22,476 | € 689
Fair Value | Revolving Credit Facility | DB Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 185,553 |  |  | 183,734 |  |
Fair Value | Revolving Credit Facility | NMNLC Credit Facility II | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 2,848 |  |  | $ 3,775 |  |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2E + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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