Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 7,820	$ 8,187
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 891,700	$ 805,700
Derivative Average Remaining Maturity	[1],[3]	4 years 9 months	4 years 11 months
Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,250	$ 1,498
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 189,500	$ 167,500
Derivative Average Remaining Maturity	[1],[3]	4 years 7 months	4 years 7 months
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 0	$ 0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	0	0
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	2,250	1,498
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 189,500	$ 167,500
Derivative Average Remaining Maturity	[1],[3]	4 years 7 months	4 years 7 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (182)	$ 168
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 281,200	$ 217,200
Derivative Average Remaining Maturity	[1],[3]	4 years 9 months	4 years 11 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 5,923	$ 6,651
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 416,000	$ 416,000
Derivative Average Remaining Maturity	[1],[3]	5 years	5 years
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 5,741	$ 6,819
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 697,200	$ 633,200
Derivative Average Remaining Maturity	[1],[3]	4 years 11 months	4 years 11 months
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	4 years	4 years 6 months
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (171)	$ (130)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	5,000	5,000
Standard & Poor's, BB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	0	0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	0	0
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	(171)	(130)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5,000	$ 5,000

[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.