Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	$ 8,187	[1]	$ 8,565	[1]
Derivative Average Remaining Maturity	4 years 11 months	[1],[2]	4 years 5 months	[1],[2]
Single Name Credit Default Swaps [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	1,498	[1]	614	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	167,500	[1],[3]	117,500	[1],[3]
Derivative Average Remaining Maturity	4 years 7 months	[1],[2]	5 years 1 month	[1],[2]
Single Name Credit Default Swaps [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	168	[1]	656	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	217,200	[1],[3]	142,200	[1],[3]
Derivative Average Remaining Maturity	4 years 11 months	[1],[2]	4 years 11 months	[1],[2]
Single Name Credit Default Swaps [Member] | Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(130)	[1]	0	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	5,000	[1],[3]	0	[1],[3]
Derivative Average Remaining Maturity	4 years 6 months	[1],[2]
Credit Default Swaps Referencing Indices [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	6,651	[1]	7,295	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	416,000	[1],[3]	405,000	[1],[3]
Derivative Average Remaining Maturity	5 years	[1],[2]	5 years	[1],[2]
Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Maximum Potential Future Exposure on Credit Risk Derivatives	805,700	[1],[3]	664,700	[1],[3]
Credit Default Subtotal [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	1,498	[1]	614	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	167,500	[1],[3]	117,500	[1],[3]
Derivative Average Remaining Maturity	4 years 7 months	[1],[2]	5 years 1 month	[1],[2]
Credit Default Subtotal [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	6,819	[1]	7,951	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	633,200	[1],[3]	547,200	[1],[3]
Derivative Average Remaining Maturity	4 years 11 months	[1],[2]	5 years	[1],[2]
Credit Default Subtotal [Member] | Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	(130)	[1]	0	[1]
Maximum Potential Future Exposure on Credit Risk Derivatives	$ 5,000	[1],[3]	$ 0	[1],[3]
Derivative Average Remaining Maturity	4 years 6 months	[1],[2]

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

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

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