Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	$ 2,122	[1]	$ (3,707)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	671,700	[1],[2]	696,000	[1],[2]
Derivative Average Remaining Maturity	4 years 7 months 6 days	[1],[3]	5 years 2 months 12 days	[1],[3]
Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(80)	[1]	(2,077)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	112,500	[1],[2]	124,500	[1],[2]
Derivative Average Remaining Maturity	5 years 2 months 12 days	[1],[3]	5 years 10 months 24 days	[1],[3]
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	[1]	0	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	0	[1],[2]	0	[1],[2]
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(80)	[1]	(2,077)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	112,500	[1],[2]	124,500	[1],[2]
Derivative Average Remaining Maturity	5 years 2 months 12 days	[1],[3]	5 years 10 months 24 days	[1],[3]
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(1,734)	[1]	(2,345)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	144,200	[1],[2]	135,500	[1],[2]
Derivative Average Remaining Maturity	5 years	[1],[3]	5 years 6 months	[1],[3]
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	3,936	[1]	937	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	415,000	[1],[2]	430,000	[1],[2]
Derivative Average Remaining Maturity	4 years 2 months 12 days	[1],[3]	5 years	[1],[3]
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	2,202	[1]	(1,408)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	559,200	[1],[2]	565,500	[1],[2]
Derivative Average Remaining Maturity	4 years 4 months 24 days	[1],[3]	5 years 1 month 6 days	[1],[3]
Standard Poors BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	[1]	(222)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	0	[1],[2]	6,000	[1],[2]
Derivative Average Remaining Maturity			4 years 6 months	[1],[3]
Standard Poors BB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	[1]	0	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	0	[1],[2]	0	[1],[2]
Standard Poors BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	0	[1]	(222)	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	$ 0	[1],[2]	$ 6,000	[1],[2]
Derivative Average Remaining Maturity			4 years 6 months	[1],[3]

[1] The rating agency designations are based on ratings from Standard and Poor’s (“S&P”).

[2] Assumes the value of the referenced credit obligations is zero.

[3] The weighted average years to maturity of the credit default swaps is calculated based on weighted average notional amounts.