Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Contractholder funds activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contractholder funds activity
Balance, beginning of year	$ 20,260	$ 21,295	$ 22,529
Deposits	1,130	1,164	1,203
Interest credited	687	722	760
Benefits	(901)	(966)	(1,077)
Surrenders and partial withdrawals	(999)	(1,053)	(1,278)
Maturities of and interest payments on institutional products	0	(86)	(1)
Contract charges	(826)	(829)	(818)
Net transfers from separate accounts	5	5	7
Other adjustments	78	8	(30)
Balance, end of year	$ 19,434	$ 20,260	$ 21,295