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Counterparty Risk (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2012
Risks and Uncertainties [Abstract]  
Investments held with dealers as collateral $ 1,100,000,000
Investments securities held as collateral 13,800,000
Unencumbered investments held in custody $ 271,700,000
Concentration risk threshold 15.00%