Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Investments held with dealers as collateral	$ 1,410,000	$ 1,150,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	10,600	33,400
Unencumbered investments held in custody	61,300
Cash and cash equivalents	$ 47,233	$ 123,274	$ 183,909	$ 114,140
Concentration risk threshold	15.00%