Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Liabilities for guarantees narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities for guarantees:
Liability for guarantees	$ 369	$ 364	$ 365
Variable annuities | In the event of death
Liabilities for guarantees:
Liability for guarantees	85	100	105
Variable annuities | Liability for guarantees related to income benefits
Liabilities for guarantees:
Liability for guarantees	26	40	64
Variable annuities | Guaranteed accumulation benefits
Liabilities for guarantees:
Liability for guarantees	22	34	38
Variable annuities | For cumulative periodic withdrawals
Liabilities for guarantees:
Liability for guarantees	12	9	14
Variable annuities | Other guarantees
Liabilities for guarantees:
Liability for guarantees	$ 224	$ 181	$ 144