Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Contractholder funds (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contractholder funds activity
Balance, beginning of year	$ 19,470	$ 20,542	$ 21,816
Deposits	909	969	1,052
Interest credited	635	672	716
Benefits	(871)	(947)	(1,060)
Surrenders and partial withdrawals	(960)	(1,014)	(1,246)
Maturities of and interest payments on institutional products	0	(86)	(1)
Contract charges	(655)	(665)	(684)
Net transfers from separate accounts	4	5	7
Other adjustments	60	(6)	(58)
Balance, end of year	$ 18,592	$ 19,470	$ 20,542