Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Risks and Uncertainties [Abstract]
Investments held with dealers as collateral	$ 1,150,000	$ 1,350,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	33,400	16,800
Unencumbered investments held in custody	119,600
Cash and Cash Equivalents, at Carrying Value	$ 123,274	$ 183,909	$ 114,140	$ 183,489
Concentration risk threshold	15.00%