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
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (762)	$ (780)
Gains (losses) included in shareholders' net income:
GMIB fair value gain/(loss)	42	11
Other	16	(4)
Total gains (losses) included in shareholders' net income	58	7
Gains (losses) included in other comprehensive income	0	0
Gains (losses) required to adjust future policy benefits for settlement annuities	0	0
Purchases, sales, settlements:
Purchases	0	0
Sales	0	0
Settlements	22	12
Total purchases, sales, and settlements	22	12
Transfers into/(out of) Level 3:
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Transfers into/(out of) Level 3	0	0
Ending balance	(682)	(761)
Total gains (losses) included in income attributable to instruments held at the reporting date	$ (58)	$ 7