Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 6,556	$ 4,837
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,348,300	$ 1,338,300
Derivative Average Remaining Maturity	[1],[3]	3 years 9 months	4 years 8 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]	$ 2,082	$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 157,000	$ 152,000
Derivative Average Remaining Maturity	[1],[3]	2 years 1 month	2 years 2 months
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 2,082	$ 1,953
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 157,000	$ 152,000
Derivative Average Remaining Maturity	[1],[3]	2 years 1 month	2 years 2 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 3,672	$ 2,930
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 358,700	$ 353,700
Derivative Average Remaining Maturity	[1],[3]	2 years	2 years 2 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 769	$ (76)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 817,600	$ 817,600
Derivative Average Remaining Maturity	[1],[3]	5 years	6 years 5 months
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 4,441	$ 2,854
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,176,300	$ 1,171,300
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month	5 years 1 month
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	6 months	8 months
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 33	$ 30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	15,000	15,000
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	33	30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 15,000	$ 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.