Counterparty Risk (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Risks and Uncertainties [Abstract]
Collateral Pledged on Reverse Repurchase Agreements Segregated under Securities Exchange Commission Regulation	$ 1,630,000,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	35,900,000	145,500,000
Unencumbered investments held in custody	224,800,000
Cash and Cash Equivalents, at Carrying Value	$ 156,250,000	$ 114,140,000	$ 170,362,000	$ 183,489,000
Concentration risk threshold	15.00%