Counterparty Risk (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Risks and Uncertainties [Abstract]
Collateral Pledged on Reverse Repurchase Agreements Segregated under Securities Exchange Commission Regulation	$ 1,940,000,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	145,500,000	13,300,000
Unencumbered investments held in custody	158,700,000
Cash and Cash Equivalents, at Carrying Value	$ 114,140,000	$ 183,489,000	$ 59,084,000	$ 62,737,000
Concentration risk threshold	15.00%