Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 8,970	$ (4,003)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 922,700	$ 893,700
Derivative Average Remaining Maturity	[1],[3]	4 years 3 months	4 years 6 months
Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,726	$ 1,689
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 150,500	$ 152,500
Derivative Average Remaining Maturity	[1],[3]	3 years 10 months	3 years 11 months
Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 7,721	$ (2,792)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 763,200	$ 731,200
Derivative Average Remaining Maturity	[1],[3]	4 years 5 months	4 years 7 months
Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (477)	$ (2,900)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 9,000	$ 10,000
Derivative Average Remaining Maturity	[1],[3]	3 years 6 months	4 years 1 month
Single Name Credit Default Swaps [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,726	$ 1,689
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 150,500	$ 152,500
Derivative Average Remaining Maturity	[1],[3]	3 years 10 months	3 years 11 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,426	$ (5,066)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 347,200	$ 315,200
Derivative Average Remaining Maturity	[1],[3]	3 years 8 months	4 years 2 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (477)	$ (2,900)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 9,000	$ 10,000
Derivative Average Remaining Maturity	[1],[3]	3 years 6 months	4 years 1 month
Credit Default Swaps Referencing Indices [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
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]	$ 6,295	$ 2,274
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]
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.