Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Risks and Uncertainties [Abstract]
Investments held with dealers as collateral	$ 1,160,000	$ 1,150,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	13,100	33,400
Unencumbered investments held in custody	79,000
Cash and Cash Equivalents, at Carrying Value	$ 111,423	$ 123,274	$ 179,618	$ 183,909
Concentration risk threshold	15.00%

Threshold used by the Company to determine disclosure of counterparty concentration risk.

No definition available.

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements which have been sold prior to period end but remain unsettled at the balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Fair value as of the balance sheet date of securities that are free and clear of any encumbrances such as reverse repurchase agreements.

+ References

No definition available.

+ Details

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- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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- References

No definition available.

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