Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	$ (537,267)	$ 65,273	$ (2,773,701)	$ 633,822
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	(1,199)	(10,755)	(4,898)	(68,720)
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	950	201	(292)	53
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	(537,079)	75,810	(2,768,959)	702,465
Short-term investments and cash equivalents
Schedule Of Gain Loss On Investments [Line Items]
Total realized investment gains (losses), net	$ 61	$ 17	$ 448	$ 24