Derivative Instruments (Details 5) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	$ (3,205,000)	$ (3,707,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	698,700,000	696,000,000
Derivative Average Remaining Maturity	5.1	5.2
Standard Poors AAA To A Minus Ratings [Member] \| Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(1,630,000)	(2,077,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	124,500,000	124,500,000
Derivative Average Remaining Maturity	5.7	5.9
Standard Poors AAA To A Minus Ratings [Member] \| Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	0
Maximum Potential Future Exposure on Credit Risk Derivatives	0	0
Derivative Average Remaining Maturity	--	--
Standard Poors AAA To A Minus Ratings [Member] \| Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(1,630,000)	(2,077,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	124,500,000	124,500,000
Derivative Average Remaining Maturity	5.7	5.9
StandardPoorsBBBRatingMember \| Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(2,030,000)	(2,345,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	138,200,000	135,500,000
Derivative Average Remaining Maturity	5.7	5.5
StandardPoorsBBBRatingMember \| Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	565,000	937,000
Maximum Potential Future Exposure on Credit Risk Derivatives	430,000,000	430,000,000
Derivative Average Remaining Maturity	4.7	5.0
StandardPoorsBBBRatingMember \| Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(1,465,000)	(1,408,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	568,200,000	565,500,000
Derivative Average Remaining Maturity	5.0	5.1
StandardPoorsBBRatingMember \| Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(110,000)	(222,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	6,000,000	6,000,000
Derivative Average Remaining Maturity	4.2	4.5
StandardPoorsBBRatingMember \| Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	0
Maximum Potential Future Exposure on Credit Risk Derivatives	0	0
Derivative Average Remaining Maturity	--	--
StandardPoorsBBRatingMember \| Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(110,000)	(222,000)
Maximum Potential Future Exposure on Credit Risk Derivatives	$ 6,000,000	$ 6,000,000
Derivative Average Remaining Maturity	4.2	4.5