Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2019		Dec. 31, 2018
Credit Derivatives [Line Items]
Document Period End Date		Jun. 30, 2019
Credit Derivative Current Fair Value	[1]	$ 6,056		$ 4,837
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,331,300		$ 1,338,300
Derivative Average Remaining Maturity	[1],[3]	3 years 9 months 18 days		4 years 8 months 12 days
Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,015		$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 147,000		$ 152,000
Derivative Average Remaining Maturity	[1],[3]	2 years 1 month 6 days		2 years 2 months 12 days
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,015		$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 147,000		$ 152,000
Derivative Average Remaining Maturity	[1],[3]	2 years 1 month 6 days		2 years 2 months 12 days
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 3,762		$ 2,930
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 321,700		$ 353,700
Derivative Average Remaining Maturity	[1],[3]	2 years		2 years 2 months 12 days
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 288		$ (76)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 852,600		$ 817,600
Derivative Average Remaining Maturity	[1],[3]	4 years 9 months 18 days		6 years 4 months 24 days
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 4,050		$ 2,854
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,174,300		$ 1,171,300
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month 6 days		5 years 1 month 6 days
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1]	3 months 18 days	[3]	8 months 12 days	[2]
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (9)		$ 30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	10,000		15,000
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	(9)		30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 10,000		$ 15,000

[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.