Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	$ (1,507,074)	$ (2,447,694)	$ (1,497,589)	$ (2,461,936)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	1,270	87,081	(1,743)	84,935
Commercial mortgage and other loans
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(241)	(1,395)	(230)	(1,401)
Joint ventures and limited partnerships
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(11)	0	(34)	0
Derivatives
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	(1,508,100)	(2,533,802)	(1,495,619)	(2,545,412)
Short-term investments
Gain (Loss) on Investments [Line Items]
Realized investment (gains) losses, net	$ 8	$ 422	$ 37	$ (58)