Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Risks and Uncertainties [Abstract]
Collateral Pledged on Reverse Repurchase Agreements Segregated under Securities Exchange Commission Regulation	$ 1,560,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	14,900	$ 145,500
Unencumbered investments held in custody	105,000
Cash and Cash Equivalents, at Carrying Value	$ 139,395	$ 114,140	$ 129,124	$ 183,489
Concentration risk threshold	15.00%