Derivative Instruments (Details 5) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	$ (3,707,000)	$ (9,936,000)
DerivativeAverageRemainingMaturity	5.2	5.2
Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Maximum Potential Future Exposure on Credit Risk Derivatives	696,000,000	614,000,000
Standard Poors AAA To A Ratings [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(2,077,000)	(1,774,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	124,500,000	85,000,000
DerivativeAverageRemainingMaturity	5.9	5.5
Standard Poors AAA To A Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(2,077,000)	(1,774,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	124,500,000	85,000,000
DerivativeAverageRemainingMaturity	5.9	5.5
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(2,345,000)	(4,267,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	135,500,000	114,000,000
DerivativeAverageRemainingMaturity	5.5	5.6
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	937,000	(3,895,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	430,000,000	415,000,000
DerivativeAverageRemainingMaturity	5.0	5.0
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(1,408,000)	(8,162,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	565,500,000	529,000,000
DerivativeAverageRemainingMaturity	5.1	5.1
Standard Poors BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(222,000)	0
Maximum Potential Future Exposure on Credit Risk Derivatives	6,000,000	0
DerivativeAverageRemainingMaturity	4.5	--
Standard Poors BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
CreditDerivativeCurrentFairValue	(222,000)	0
Maximum Potential Future Exposure on Credit Risk Derivatives	$ 6,000,000	$ 0
DerivativeAverageRemainingMaturity	4.5	--