Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (4,003)	$ 8,187
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 893,700	$ 805,700
Derivative Average Remaining Maturity	[1],[3]	4 years 6 months	4 years 11 months
Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,689	$ 1,498
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,500	$ 167,500
Derivative Average Remaining Maturity	[1],[3]	3 years 11 months	4 years 7 months
Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (2,792)	$ 6,819
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 731,200	$ 633,200
Derivative Average Remaining Maturity	[1],[3]	4 years 7 months	4 years 11 months
Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (2,900)	$ (130)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 10,000	$ 5,000
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	4 years 6 months
Single Name Credit Default Swaps [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,689	$ 1,498
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,500	$ 167,500
Derivative Average Remaining Maturity	[1],[3]	3 years 11 months	4 years 7 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (5,066)	$ 168
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 315,200	$ 217,200
Derivative Average Remaining Maturity	[1],[3]	4 years 2 months	4 years 11 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (2,900)	$ (130)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 10,000	$ 5,000
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	4 years 6 months
Credit Default Swaps Referencing Indices [Member] | Standard Poors AAA To A Ratings [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
Derivative Average Remaining Maturity	[1],[3]	0 years	0 years
Credit Default Swaps Referencing Indices [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,274	$ 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
Credit Default Swaps Referencing Indices [Member] | Standard & Poor's, BB+ Rating [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
Derivative Average Remaining Maturity	[1],[3]	0 years	0 years

[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.