Fair Value Measurements (Narrative) (Details)	12 Months Ended
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

Fair Value Measurement [Line Items]

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Number of knowledgeable outside security brokers consulted to determine fair value.

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Percentage of unadjusted fair value obtained

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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

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Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Section 55
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Estimate remaining period the underlying assets are expected to be liquidated by the investees for investments for which net asset value per share is calculated (including by unit, membership interest, or other ownership interest unit measure), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (b)
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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
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-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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