Fair Value Measurements - Changes in Level 3 Financial Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities And Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 776	$ 726
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	0	0
Other	25	(18)
Total gains (losses) included in shareholders' net income	25	(18)
Gains (losses) included in other comprehensive income	(11)	(1)
Gains (losses) required to adjust future policy benefits for settlement annuities	7	29
Purchases, sales, and settlements:
Purchases	133	96
Sales	(95)	(140)
Settlements	(74)	(74)
Total purchases, sales, settlements	(36)	(118)
Transfers into/(out of) Level 3:
Transfers into Level 3	275	338
Transfers out of Level 3	(304)	(180)
Total transfers into/(out of) Level 3	(29)	158
Ending Balance	732	776
Total gains (losses) included in income attributable to instruments held at the reporting date	(9)	(18)
Derivative Financial Instruments, Assets [Member] | Guaranteed Minimum Income Benefit [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	799	907
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	31	(47)
Other	1	0
Total gains (losses) included in shareholders' net income	32	(47)
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	(54)	(61)
Total purchases, sales, settlements	(54)	(61)
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	777	799
Total gains (losses) included in income attributable to instruments held at the reporting date	$ 32	$ (47)