Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 4,837	$ 7,572
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,338,300	$ 951,300
Derivative Average Remaining Maturity	[1],[3]	4 years 8 months	3 years 5 months
Standard Poors AAA To A Ratings [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 1,953	$ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,000	$ 162,000
Derivative Average Remaining Maturity	[1],[3]	2 years 2 months	2 years 11 months
Standard & Poor's, BBB Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 2,854	$ 4,414
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,171,300	$ 784,300
Derivative Average Remaining Maturity	[1],[3]	5 years 1 month	3 years 6 months
Standard & Poor's, BB+ Rating [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	[1]	$ 30	$ 30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 15,000	$ 5,000
Derivative Average Remaining Maturity	[1],[3]	8 months	1 year 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]	$ 1,953	$ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 152,000	$ 162,000
Derivative Average Remaining Maturity	[1],[3]	2 years 2 months	2 years 11 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]	$ 2,930	$ 4,469
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 353,700	$ 361,700
Derivative Average Remaining Maturity	[1],[3]	2 years 2 months	2 years 11 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	$ 30
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 15,000	$ 5,000
Derivative Average Remaining Maturity	[1],[3]	8 months	1 year 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]	$ (76)	$ (55)
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 817,600	$ 422,600
Derivative Average Remaining Maturity	[1],[3]	6 years 5 months	4 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.