Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (36,890)	$ 276,590	$ (85,932)	$ 244,011
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(3,318)	4,230	(4,127)	200
Equity securities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	0	(11)	0	(125)
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(125)	(349)	6	(488)
LPS/LLCS
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	2	(28)	849	(101)
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(33,448)	272,748	(82,641)	244,531
Short-term investments and cash equivalents
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (1)	$ 0	$ (19)	$ (6)