Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (757,691)	$ (1,342,219)
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	6,422	(731)
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(342)	(781)
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(764,540)	(1,340,928)
Other invested assets
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	500	0
Short-term investments and cash equivalents
Schedule Of Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ 269	$ 221