Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 732	$ 776
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	0	0
Other	(20)	23
Total gains (losses) included in shareholders' net income	(20)	23
Gains (losses) included in other comprehensive income	(5)	(8)
Gains (losses) required to adjust future policy benefits for settlement annuities	(4)	0
Purchases, sales, and settlements:
Purchases	10	25
Sales	(11)	(47)
Settlements	(2)	(27)
Total purchases, sales, settlements	(3)	(49)
Transfers into/(out of) Level 3:
Transfers into Level 3	20	40
Transfers out of Level 3	(124)	(55)
Total transfers into/(out of) Level 3	(104)	(15)
Ending Balance	596	727
Total gains (losses) included in income attributable to instruments held at the reporting date	(7)	(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	777	799
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	(40)	(11)
Other	(3)	1
Total gains (losses) included in shareholders' net income	(43)	(10)
Gains (losses) included in other comprehensive income	0	0
Gains (losses) required to adjust future policy benefits for settlement annuities	0	0
Purchases, sales, and settlements:
Purchases	0	0
Sales	0	0
Settlements	(17)	(12)
Total purchases, sales, settlements	(17)	(12)
Transfers into/(out of) Level 3:
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total transfers into/(out of) Level 3	0	0
Ending Balance	717	777
Total gains (losses) included in income attributable to instruments held at the reporting date	$ (43)	$ (10)