Investments (Realized Investment Gains Losses Net) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ (805,400)	$ (3,442,760)	$ 6,052
Fixed maturities
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(3,177)	86,184	4,831
Commercial mortgage and other loans
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(840)	(2,326)	(161)
Derivatives
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(801,429)	(3,526,514)	1,381
Other long-term investments
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	(39)	(648)	1
Short-term Investments
Gain Loss On Investments [Line Items]
Realized investment gains (losses), net	$ 85	$ 544	$ 0