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Fair Value Measurements (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) broker
Fair Value Measurement [Line Items] |  |
Percentage of portfolio of unadjusted fair values obtained | 98.30% |
Two Private Equity Funds using NAV | $ 78,200,000 |
One Private Equity Fund using NAV | $ 1,200,000 |
Minimum [Member] |  |
Fair Value Measurement [Line Items] |  |
Number of knowledgeable outside security brokers consulted to determine fair value | broker |  | 1
Liquidating investment, remaining period | 1 year |
Maximum [Member] |  |
Fair Value Measurement [Line Items] |  |
Liquidating investment, remaining period | 9 years |
Unsecured Debt [Member] |  |
Fair Value Measurement [Line Items] |  |
Debt instrument, face amount | $ 375,000,000 | $ 375,000,000
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets, fair value | 4,734,995,000 | 4,317,826,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets, fair value | 0 | 1,203,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets, fair value | 3,579,308,000 | 3,156,091,000
Commercial Mortgage Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Holings in mortgage backed securities | 17,600,000 | $ 18,900,000
Equity Securities [Member] | Private Equity Funds Net Asset Value [Member] |  |
Fair Value Measurement [Line Items] |  |
Unfunded commitments | $ 0 |

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