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Fair Value Measurement (Narrative) (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) broker | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Percentage of portfolio of unadjusted fair values obtained |  |  | 98.40% |  |
Two private equity funds using NAV | $ 71,400,000 |  | $ 71,400,000 |  |
One private equity fund using NAV | 1,200,000 |  | 1,200,000 |  |
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount | 0 | $ 0 | $ 0 | $ 0 |
Minimum [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Number of knowledgeable outside security brokers consulted to determine fair value | broker |  |  | 1 |  |
Fair Value, liquidating investment, remaining period |  |  | 1 year |  |
Maximum [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Fair Value, liquidating investment, remaining period |  |  | 10 years |  |
Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Unsecured notes | 375,000,000 |  | $ 375,000,000 |  | $ 375,000,000
Commercial Mortgage Back Securities [Member] | Level 2 [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Debt securities, trading, and equity securities, FV-NI | 12,000,000.0 |  | 12,000,000.0 |  | 18,900,000
Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 4,554,532,000 |  | 4,554,532,000 |  | 4,317,826,000
Liabilities, fair value | 1,356,000 |  | 1,356,000 |  | 77,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 3,513,548,000 |  | 3,513,548,000 |  | 3,156,091,000
Liabilities, fair value | 0 |  | 0 |  | 0
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Liabilities, fair value | 406,144,000 |  | 406,144,000 |  | 394,279,000
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Liabilities, fair value | $ 406,144,000 |  | $ 406,144,000 |  | 394,279,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure |  |  |  |  | $ 1,200,000

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