Counterparty Risk (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Risks and Uncertainties [Abstract]
Collateral Pledged on Reverse Repurchase Agreements Segregated under Securities Exchange Commission Regulation	$ 1,350,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	16,800	$ 145,500
Unencumbered investments held in custody	75,200
Cash and Cash Equivalents, at Carrying Value	$ 183,909	$ 114,140	$ 183,489	$ 59,084
Concentration risk threshold	15.00%