Investments (Changes in Net Unrealized Investment Gains Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes In Net Unrealized Investment Gains Losses Included In Accumulated Other Comprehensive Loss [Abstract]
Balance at January 1,	$ 1,277	$ 1,620	$ 2,628
Fixed maturity securities on which noncredit OTTI losses have been recognized	1	24	15
Unrealized investment gains (losses) during the year	1,938	260	(2,303)
Unrealized investment gains (losses) relating to
Future policy benefits	(1,448)	(796)	487
DAC and VOBA related to noncredit OTTI losses recognized in AOCI	0	(1)	1
DAC, VOBA and DSI	(67)	5	208
Deferred income tax benefit (expense) related to noncredit OTTI losses recognized in AOCI	1	(9)	(5)
Deferred income tax benefit (expense)	158	174	589
Balance at December 31,	1,860	1,277	1,620
Change in net unrealized investment gains (losses)	$ 583	$ (343)	$ (1,008)