Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ 727	$ 792	$ 776	$ 726
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)		0	0	0	0
Other		1	9	24	(16)
Total gains (losses) included in shareholders' net income		1	9	24	(16)
Gains (losses) included in other comprehensive income		5	(1)	(3)	(1)
Gains (losses) required to adjust future policy benefits for settlement annuities	[1]	9	24	9	35
Purchases, sales, and settlements:
Purchases		65	23	90	47
Sales		(23)	(125)	(70)	(125)
Settlements		(12)	(5)	(39)	(16)
Total purchases, sales, settlements		30	(107)	(19)	(94)
Transfers into/(out of) Level 3:
Transfers into Level 3		36	63	76	191
Transfers out of Level 3		(57)	(37)	(112)	(98)
Total transfers into/(out of) Level 3		(21)	26	(36)	93
Ending Balance		751	743	751	743
Total gains (losses) included in income attributable to instruments held at the reporting date		0	(1)	(6)	(23)
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	957	799	907
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)		15	32	4	93
Other		0	0	1	(1)
Total gains (losses) included in shareholders' net income		15	32	5	92
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		(15)	(14)	(27)	(24)
Total purchases, sales, settlements		(15)	(14)	(27)	(24)
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		777	975	777	975
Total gains (losses) included in income attributable to instruments held at the reporting date		$ 15	$ 32	$ 5	$ 92

[1] Amounts do not accrue to shareholders.