Fair Value Measurements (Narrative) (Details)	12 Months Ended
Dec. 31, 2017 USD ($) broker	Dec. 31, 2016 USD ($) broker
Fair Value Measurement [Line Items]
Percentage of portfolio of unadjusted fair values obtained	98.00%
Fair value of assets measured on a recurring basis	$ 3,738,293,000	$ 3,548,227,000
Liabilities fair value	1,323,000	785,000
Notes payable	371,335,000	320,000,000
Level 2 [Member]
Fair Value Measurement [Line Items]
Fair value of assets measured on a recurring basis	2,921,995,000	2,854,987,000
Liabilities fair value	1,200,000	765,000
Level 2 [Member] \| Commercial Mortgage Backed Securities [Member]
Fair Value Measurement [Line Items]
Trading securities, fair value disclosure	$ 19,300,000	$ 30,000,000
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]
Liabilities fair value	$ 375,000,000
Secured Notes One [Member]
Fair Value Measurement [Line Items]
Secured loan		$ 120,000,000
Secured Notes One [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Secured loan	0	120,000,000
Secured Notes Two [Member]
Fair Value Measurement [Line Items]
Secured loan		20,000,000
Secured Notes Two [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Secured loan	0	20,000,000
Equity Securities [Member]
Fair Value Measurement [Line Items]
Fair value of assets measured on a recurring basis	537,240,000	357,327,000
Equity Securities [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Fair value of assets measured on a recurring basis	34,869,000	$ 31,809,000
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	5 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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This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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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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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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Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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