Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Securities Collateral relating to Reverse Repurchase Agreements	$ 1,120,000	$ 1,360,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	20,800	16,800
Unencumbered investments held in custody	76,100
Cash and Cash Equivalents, at Carrying Value	$ 179,618	$ 183,909	$ 139,395	$ 114,140
Concentration risk threshold	15.00%