Investments (excluding Consolidated Investment Entities) - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	$ (227)	$ (363)	$ (560)
After-tax net realized capital gains (losses), after tax	(120)	(268)	(370)
Proceeds from sale of investments
Proceeds on sales	4,905	4,742	4,932
Gross gains	93	91	91
Gross losses	56	157	104
Fixed maturities, available-for-sale, including securities pledged
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	7	(98)	(90)
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	(282)	(296)	(336)
Equity securities, available-for-sale
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	(1)	1	(4)
Derivatives
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	98	32	(68)
Other investments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	(9)	8	0
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	(18)	(19)	(16)
Guaranteed benefit derivatives
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total net realized capital gains (losses)	$ (22)	$ 9	$ (46)