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Fair Value Measurement (Narrative) (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2022 | Sep. 30, 2021 | Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percentage of portfolio of unadjusted fair values obtained | 97.80% |  |
Three private equity funds using NAV | $ 92,100,000 |  |
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount | $ 0 | $ 0 |
Minimum |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, liquidating investment, remaining period | 1 year |  |
Maximum |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, liquidating investment, remaining period | 10 years |  |
Unsecured debt | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Unsecured notes | $ 375,000,000 |  | $ 375,000,000
Commercial Mortgage Back Securities [Member] | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, trading, and equity securities, FV-NI | 27,800,000 |  | 25,200,000
Private Equity Funds [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Unfunded commitments to VIEs | 10,000,000 |  | 32,000,000
Venture Capital Investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
One private equity fund using NAV | 100,000 |  |
Technology, Healthcare, Specialty Customer Goods and Service |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
One private equity fund using NAV | 600,000 |  |
Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets, fair value disclosure | 4,682,815,000 |  | 5,142,589,000
Liabilities | 169,000 |  | 301,000
Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets, fair value disclosure | 3,892,574,000 |  | 4,099,687,000
Liabilities | $ 0 |  | $ 0

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