Investments (Realized Investment Gains Losses Net) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	$ 758,449	$ (208,538)	$ 114,067
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	66,110	9,505	17,563
Equity securities
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	(1,570)	397	1,085
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	29,584	1,503	4,644
Joint ventures and limited partnerships
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	(229)	320	210
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	664,533	(220,292)	90,556
Short-term investments and cash equivalents
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	$ 21	$ 29	$ 9