Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Summary of liabilities for guarantees (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	$ 365	$ 366
Less reinsurance recoverables	184	222
Net balance at beginning of period	181	144
Incurred guarantee benefits	45	37
Paid guarantee benefits	(2)	0
Net change	43	37
Net balance at end of period	224	181
Plus reinsurance recoverables	146	184
Balance at end of period	370	365
In the event of death | Variable annuities
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	100	105
Balance at end of period	85	100
At annuitization (includes income benefit guarantees) | Variable annuities
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	40	65
Balance at end of period	26	40
Accumulation at specified dates | Variable annuities
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	34	38
Balance at end of period	22	34
For cumulative periodic withdrawals | Variable annuities
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	9	14
Balance at end of period	12	9
Other Guarantees | Variable annuities
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	182	144
Balance at end of period	225	182
Liability for guarantees related to death benefits and interest-sensitive life products
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	244	223
Less reinsurance recoverables	101	106
Net balance at beginning of period	143	117
Incurred guarantee benefits	34	26
Paid guarantee benefits	(2)	0
Net change	32	26
Net balance at end of period	175	143
Plus reinsurance recoverables	87	101
Balance at end of period	262	244
At annuitization (includes income benefit guarantees)
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	44	68
Less reinsurance recoverables	40	64
Net balance at beginning of period	4	4
Incurred guarantee benefits	0	0
Paid guarantee benefits	0	0
Net change	0	0
Net balance at end of period	4	4
Plus reinsurance recoverables	25	40
Balance at end of period	29	44
Liability for guarantees related to accumulation and withdrawal benefits
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	77	75
Less reinsurance recoverables	43	52
Net balance at beginning of period	34	23
Incurred guarantee benefits	11	11
Paid guarantee benefits	0	0
Net change	11	11
Net balance at end of period	45	34
Plus reinsurance recoverables	34	43
Balance at end of period	$ 79	$ 77