Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 596	$ 727	$ 732	$ 776
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	0	0	0	0
Other	(1)	1	(21)	24
Total gains (losses) included in shareholders' net income	(1)	1	(21)	24
Gains (losses) included in other comprehensive income	(5)	5	(10)	(3)
Gains (losses) required to adjust future policy benefits for settlement annuities	(2)	9	(6)	9
Purchases, sales, and settlements:
Purchases	6	65	16	90
Sales	0	(23)	(11)	(70)
Settlements	(13)	(12)	(15)	(39)
Total purchases, sales, settlements	(7)	30	(10)	(19)
Transfers into/(out of) Level 3:
Transfers into Level 3	0	36	20	76
Transfers out of Level 3	(53)	(57)	(177)	(112)
Total transfers into/(out of) Level 3	(53)	(21)	(157)	(36)
Ending Balance	528	751	528	751
Total gains (losses) included in income attributable to instruments held at the reporting date	(1)	0	(8)	(6)
Fixed Maturities And Equity Securities [Member] | Accounting Standards Update 2016-01 [Member]
Transfers into/(out of) Level 3:
Transfers out of Level 3			(70)
Derivative Financial Instruments, Assets [Member] | Guaranteed Minimum Income Benefit [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	717	777	777	799
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	(27)	15	(67)	4
Other	1	0	(2)	1
Total gains (losses) included in shareholders' net income	(26)	15	(69)	5
Gains (losses) included in other comprehensive income	0	0	0	0
Gains (losses) required to adjust future policy benefits for settlement annuities	0	0	0	0
Purchases, sales, and settlements:
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(7)	(15)	(24)	(27)
Total purchases, sales, settlements	(7)	(15)	(24)	(27)
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	684	777	684	777
Total gains (losses) included in income attributable to instruments held at the reporting date	$ (26)	$ 15	$ (69)	$ 5