Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2019		Dec. 31, 2018
Credit Derivatives [Line Items]
Document Period End Date		Sep. 30, 2019
Credit Derivative Current Fair Value	[1]	$ 5,749		$ 4,837
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,337,800		$ 1,338,300
Derivative Average Remaining Maturity	[1],[3]	3 years 10 months 24 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]	$ 1,018		$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 117,500		$ 152,000
Derivative Average Remaining Maturity	[1],[3]	1 year 7 months 6 days		2 years 2 months 12 days
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 342		$ 0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 240,000		$ 0
Derivative Average Remaining Maturity	[1],[3]	10 years 2 months 12 days		0 years
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1,360		$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 357,500		$ 152,000
Derivative Average Remaining Maturity	[1],[3]	7 years 4 months 24 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]	$ 4,103		$ 2,930
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 337,700		$ 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]	$ 111		$ (76)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 632,600		$ 817,600
Derivative Average Remaining Maturity	[1],[3]	2 years 10 months 24 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]	$ 4,214		$ 2,854
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 970,300		$ 1,171,300
Derivative Average Remaining Maturity	[1],[3]	2 years 7 months 6 days		5 years 1 month 6 days
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1]	2 years 8 months 12 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]	$ 175		$ 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]	175		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.