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Fair Value Measurement (Narrative) (Details) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Percentage of portfolio of unadjusted fair values obtained | 98.20% |  |
Three private equity funds using NAV | $ 89,200,000 |  |
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount | 0 | $ 0 |
Unsecured Notes Two | Revolving Credit Facility |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Drawn down amount under credit facility | $ 200,000,000 |  | $ 25,000,000
Minimum |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair Value, liquidating investment, remaining period | 1 year |  |
Maximum |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair Value, liquidating investment, remaining period | 9 years |  |
Technology, Healthcare, Specialty Customer Goods and Service |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
One private equity fund using NAV | $ 0 |  |
Venture Capital Investments |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
One private equity fund using NAV | 1,700,000 |  |
Private Equity Funds |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Unfunded commitments to VIEs | 8,000,000 |  | 9,000,000
Level 2 | Unsecured debt |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Unsecured notes | 375,000,000 |  | 375,000,000
Level 2 | Commercial Mortgage Back Securities |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt securities, trading, and equity securities, FV-NI | $ 25,500,000 |  | $ 27,300,000

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 835 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482900/835-30-50-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69B -Publisher FASB -URI https://asc.fasb.org//1943274/2147481568/470-20-55-69B Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69C -Publisher FASB -URI https://asc.fasb.org//1943274/2147481568/470-20-55-69C Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482925/835-30-45-2 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482949/835-30-55-8 + Details
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