Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	$ (1,342,219)	$ 558,690
Fixed maturities
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	(731)	(73,799)
Commercial mortgage and other loans
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	(781)	(620)
LPs/LLCs
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	0	0
Derivatives
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	(1,340,928)	633,120
Short-term investments and cash equivalents
Schedule Of Gain Loss On Investments [Line Items]
Realized investment (gains) losses, net	$ 221	$ (11)