Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 528	$ 751	$ 732	$ 776
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	0	0	0	0
Other	0	(2)	(21)	22
Total gains (losses) included in shareholders' net income	0	(2)	(21)	22
Gains (losses) included in other comprehensive income	2	(9)	(8)	(12)
Gains (losses) required to adjust future policy benefits for settlement annuities	(2)	(2)	(8)	7
Purchases, sales, and settlements:
Purchases	3	37	19	127
Sales	0	(3)	(11)	(73)
Settlements	(14)	(30)	(29)	(69)
Total purchases, sales, settlements	(11)	4	(21)	(15)
Transfers into/(out of) Level 3:
Transfers into Level 3	24	178	44	254
Transfers out of Level 3	(73)	(116)	(250)	(228)
Total transfers into/(out of) Level 3	(49)	62	(206)	26
Ending Balance	468	804	468	804
Total gains (losses) included in income attributable to instruments held at the reporting date	(1)	1	(9)	0
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	684	777	777	799
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	(39)	40	(106)	44
Other	2	(1)	0	0
Total gains (losses) included in shareholders' net income	(37)	39	(106)	44
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	(11)	(18)	(35)	(45)
Total purchases, sales, settlements	(11)	(18)	(35)	(45)
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	636	798	636	798
Total gains (losses) included in income attributable to instruments held at the reporting date	$ (37)	$ 39	$ (106)	$ 44