Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 7,572	$ 8,970
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 951,300	$ 922,700
Derivative Average Remaining Maturity	[1],[3]	3 years 5 months	4 years 3 months
Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 3,128	$ 1,726
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 162,000	$ 150,500
Derivative Average Remaining Maturity	[1],[3]	2 years 11 months	3 years 10 months
Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 4,414	$ 7,721
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 784,300	$ 763,200
Derivative Average Remaining Maturity	[1],[3]	3 years 6 months	4 years 5 months
Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 30	$ (477)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5,000	$ 9,000
Derivative Average Remaining Maturity	[1],[3]	1 year 6 months	3 years 6 months
Single Name Credit Default Swaps [Member] | Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 3,128	$ 1,726
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 162,000	$ 150,500
Derivative Average Remaining Maturity	[1],[3]	2 years 11 months	3 years 10 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 4,469	$ 1,426
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 361,700	$ 347,200
Derivative Average Remaining Maturity	[1],[3]	2 years 11 months	3 years 8 months
Single Name Credit Default Swaps [Member] | Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 30	$ (477)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5,000	$ 9,000
Derivative Average Remaining Maturity	[1],[3]	1 year 6 months	3 years 6 months
Credit Default Swaps Referencing Indices [Member] | Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ (55)	$ 6,295
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 422,600	$ 416,000
Derivative Average Remaining Maturity	[1],[3]	4 years	5 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.