Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 6,603	$ 7,572
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 928,300	$ 951,300
Derivative Average Remaining Maturity	[1],[3]	3 years	3 years 5 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]	$ 2,574	$ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,000	$ 162,000
Derivative Average Remaining Maturity	[1],[3]	2 years 8 months	2 years 11 months
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,574	$ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,000	$ 162,000
Derivative Average Remaining Maturity	[1],[3]	2 years 8 months	2 years 11 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 4,092	$ 4,469
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 338,700	$ 361,700
Derivative Average Remaining Maturity	[1],[3]	2 years 7 months	2 years 11 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (59)	$ (55)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 422,600	$ 422,600
Derivative Average Remaining Maturity	[1],[3]	3 years 6 months	4 years
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 4,033	$ 4,414
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 761,300	$ 784,300
Derivative Average Remaining Maturity	[1],[3]	3 years 1 month	3 years 6 months
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	1 year 2 months	1 year 6 months
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (4)	$ 30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	15,000	5,000
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	(4)	30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 15,000	$ 5,000

[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.