Fair Value Measurement (Narrative) (Details)	9 Months Ended
Sep. 30, 2017 USD ($) broker	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained	97.90%
Percentage of portfolio of specific unadjusted broker quotes obtained	0.20%
Commercial Mortgage Backed Securities [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, fair value disclosure	$ 24,400,000	$ 30,000,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of knowledgeable outside security brokers consulted to determine fair value \| broker	1
Borrowings [Member] \| Unsecured debt [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unsecured notes	$ 375,000,000	180,000,000
Secured Notes One [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Secured notes	0	120,000,000
Secured Notes Two [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Secured notes	0	$ 20,000,000
Private Equity Funds, Net Asset Value [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments	$ 0
Private Equity Funds, Net Asset Value [Member] \| Equity Securities [Member] \| Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquidating investment, remaining period	1 year
Private Equity Funds, Net Asset Value [Member] \| Equity Securities [Member] \| Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquidating investment, remaining period	5 years

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

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- Definition

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-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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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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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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