Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ 751	$ 743	$ 776	$ 726
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)		0	0	0	0
Other		(2)	0	22	(16)
Total gains (losses) included in shareholders' net income		(2)	0	22	(16)
Gains (losses) included in other comprehensive income		(9)	12	(12)	11
Gains (losses) required to adjust future policy benefits for settlement annuities	[1]	(2)	0	7	35
Purchases, sales, and settlements:
Purchases		37	20	127	67
Sales		(3)	(1)	(73)	(126)
Settlements		(30)	(55)	(69)	(71)
Total purchases, sales, settlements		4	(36)	(15)	(130)
Transfers into/(out of) Level 3:
Transfers into Level 3		178	44	254	235
Transfers out of Level 3		(116)	(30)	(228)	(128)
Total transfers into/(out of) Level 3		62	14	26	107
Ending Balance		804	733	804	733
Total gains (losses) included in income attributable to instruments held at the reporting date		1	0	0	(11)
Derivative Financial Instruments, Assets [Member] | Guaranteed Minimum Income Benefit [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		777	975	799	907
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)		40	(22)	44	71
Other		(1)	1	0	0
Total gains (losses) included in shareholders' net income		39	(21)	44	71
Gains (losses) included in other comprehensive income		0	0	0	0
Gains (losses) required to adjust future policy benefits for settlement annuities	[1]	0	0	0	0
Purchases, sales, and settlements:
Purchases		0	0	0	0
Sales		0	0	0	0
Settlements		(18)	(23)	(45)	(47)
Total purchases, sales, settlements		(18)	(23)	(45)	(47)
Transfers into/(out of) Level 3:
Transfers into Level 3		0	0	0	0
Transfers out of Level 3		0	0	0	0
Total transfers into/(out of) Level 3		0	0	0	0
Ending Balance		798	931	798	931
Total gains (losses) included in income attributable to instruments held at the reporting date		$ 39	$ (21)	$ 44	$ 71

[1] Amounts do not accrue to shareholders.