Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Collateral Pledged on Reverse Repurchase Agreements Segregated under Securities Exchange Commission Regulation	$ 1,240,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	21,800	$ 16,800
Unencumbered investments held in custody	84,200
Cash and Cash Equivalents, at Carrying Value	$ 140,358	$ 183,909	$ 123,856	$ 114,140
Concentration risk threshold	15.00%