Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 5,661	$ 8,970
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 931,700	$ 922,700
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	4 years 3 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]	$ 2,704	$ 1,726
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 155,500	$ 150,500
Derivative Average Remaining Maturity	[1],[3]	3 years 7 months	3 years 10 months
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,704	$ 1,726
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 155,500	$ 150,500
Derivative Average Remaining Maturity	[1],[3]	3 years 7 months	3 years 10 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,822	$ 1,426
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 355,200	$ 347,200
Derivative Average Remaining Maturity	[1],[3]	3 years 6 months	3 years 8 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 126	$ 6,295
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 416,000	$ 416,000
Derivative Average Remaining Maturity	[1],[3]	4 years 8 months	5 years
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,948	$ 7,721
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 771,200	$ 763,200
Derivative Average Remaining Maturity	[1],[3]	4 years 2 months	4 years 5 months
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	2 years 2 months	3 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]	$ 9	$ (477)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	5,000	9,000
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	9	(477)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5,000	$ 9,000

[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.