Investments (Realized Investment Gains Losses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	$ 9,859	$ (1,507,074)	$ 568,549	$ (1,497,589)
Fixed maturities
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	15,834	1,270	(57,965)	(1,743)
Commercial mortgage and other loans
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	472	(241)	(148)	(230)
LPs/LLCs
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	0	(11)	0	(34)
Derivatives
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	(6,465)	(1,508,100)	626,655	(1,495,619)
Short-term investments and cash equivalents
Gain (Loss) on Securities [Line Items]
Realized investment (gains) losses, net	$ 18	$ 8	$ 7	$ 37