Investments (Realized Investment Gains Losses Net) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (80,186)	$ 758,449	$ (208,538)
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	27,123	66,110	9,505
Equity securities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(125)	(1,570)	397
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(337)	29,584	1,503
Joint Ventures and limited partnerships
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(221)	(229)	320
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(106,625)	664,533	(220,292)
Short-term Investments
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (1)	$ 21	$ 29