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Fair Value Measurements (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2019 USD ($) broker | Dec. 31, 2018 USD ($) broker
Fair Value Measurement [Line Items] |  |
Percentage of portfolio of unadjusted fair values obtained | 97.90% |
Minimum [Member] |  |
Fair Value Measurement [Line Items] |  |
Number of knowledgeable outside security brokers consulted to determine fair value | broker | 1 | 1
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 | 4,317,826,000 | 3,773,648,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets, fair value | 1,203,000 | 1,445,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets, fair value | 3,156,091,000 | 3,013,932,000
Commercial Mortgage Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Holings in mortgage backed securities | 18,900,000 | $ 24,800,000
Equity Securities [Member] | Private Equity Funds Net Asset Value [Member] |  |
Fair Value Measurement [Line Items] |  |
Unfunded commitments | $ 0 |
Equity Securities [Member] | Private Equity Funds Net Asset Value [Member] | Minimum [Member] |  |
Fair Value Measurement [Line Items] |  |
Liquidating investment, remaining period | 1 year |
Equity Securities [Member] | Private Equity Funds Net Asset Value [Member] | Maximum [Member] |  |
Fair Value Measurement [Line Items] |  |
Liquidating investment, remaining period | 10 years |

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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