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Counterparty Risk (Narrative) (Details) (USD $)
9 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Dec. 31, 2012
Risks and Uncertainties [Abstract]        
Cash and Cash Equivalents, at Carrying Value $ 129,124,000 $ 183,489,000 $ 186,737,000 $ 59,084,000
Investments held with dealers as collateral 1,560,000,000 1,460,000,000    
Unencumbered investments held in custody 332,200,000      
Concentration risk threshold 15.00%      
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled $ 27,000,000 $ 13,300,000