Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	$ 1,170,077	$ (35,725)	$ (327,512)	$ (2,497,661)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(2,664)	9,899	(4,407)	94,834
Commercial mortgage and other loans
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(430)	(422)	(660)	(1,823)
Joint ventures and limited partnerships
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(3)	(167)	(37)	(649)
Derivatives
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	1,173,163	(45,139)	(322,456)	(2,590,550)
Short-term investments
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	$ 11	$ 104	$ 48	$ 527