Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 985	$ (4,003)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 855,700	$ 893,700
Derivative Average Remaining Maturity	[1],[3]	4 years 8 months	4 years 6 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]	$ 1,351	$ 1,689
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 130,000	$ 152,500
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	3 years 11 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]	1,351	1,689
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 130,000	$ 152,500
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	3 years 11 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (4,570)	$ (5,066)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 304,700	$ 315,200
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	4 years 2 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 4,423	$ 2,274
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 416,000	$ 416,000
Derivative Average Remaining Maturity	[1],[3]	5 years 2 months	5 years
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (147)	$ (2,792)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 720,700	$ 731,200
Derivative Average Remaining Maturity	[1],[3]	4 years 9 months	4 years 7 months
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	3 years 2 months	4 years 1 month
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (219)	$ (2,900)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	5,000	10,000
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]	(219)	(2,900)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5,000	$ 10,000

[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.