Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	$ 884,073	$ (805,400)	$ (3,442,760)
Fixed maturities
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	(83,731)	(3,177)	86,184
Commercial mortgage and other loans
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	128	(840)	(2,326)
LPs/LLCs
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	0	(39)	(648)
Derivatives
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	967,503	(801,429)	(3,526,514)
Other invested assets
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	123	0	0
Short-term investments and cash equivalents
Schedule of Gain (Loss) on Investments [Line Items]
Realized investment gains (losses), net	$ 50	$ 85	$ 544