Fair Value Measurements - Changes in Level 3 Financial Liabilities (Details) - Derivative Financial Instruments, Liabilities [Member] - Guaranteed Minimum Income Benefit [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (653)	$ (764)	$ (762)	$ (780)
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	39	(40)	108	(44)
Other	(10)	6	1	(1)
Total gains (losses) included in shareholders' net income	29	(34)	109	(45)
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, settlements:
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	11	18	40	45
Total purchases, sales, and settlements	11	18	40	45
Transfers into/(out of) Level 3:
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Transfers into/(out of) Level 3	0	0	0	0
Ending balance	(613)	(780)	(613)	(780)
Total gains (losses) included in income attributable to instruments held at the reporting date	$ 29	$ (34)	$ 109	$ (45)