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

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

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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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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).

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