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Derivative Instruments (Details 7) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Credit Exposure of Derivative Contracts    
Estimated fair value of derivatives in net asset (liability) position $ 133,157,000 $ 136,558,000
Cash provided as collateral (6,250,000) (27,867,000)
Securities provided as collateral (14,118,000) (1,565,000)
Cash received as collateral (126,329,000) (136,414,000)
Securities received as collateral (17,664,000) (22,458,000)
Net credit exposure 9,532,000 7,118,000
Margin account related to exchange-traded futures $ 3,529,000 $ 5,605,000