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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (682)	$ (761)	$ (762)	$ (780)
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	27	(15)	69	(4)
Other	(5)	(3)	11	(7)
Total gains (losses) included in shareholders' net income	22	(18)	80	(11)
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	7	15	29	27
Total purchases, sales, and settlements	7	15	29	27
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	(653)	(764)	(653)	(764)
Total gains (losses) included in income attributable to instruments held at the reporting date	$ 22	$ (18)	$ 80	$ (11)