Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Liabilities for guarantees (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Liabilities for guarantees:
Liabilities for Guarantees on Long-Duration Contracts, Incurred Benefits	$ 45	$ 37
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	364	365
Less reinsurance recoverables	184	222
Net balance at beginning of period	180	143
Paid guarantee benefits	(2)	0
Net change	43	37
Net balance at end of period	223	180
Plus reinsurance recoverables	146	184
Balance at end of period	369	364
Liability for guarantees related to death benefits and interest-sensitive life products
Liabilities for guarantees:
Liabilities for Guarantees on Long-Duration Contracts, Incurred Benefits	33	26
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
Paid guarantee benefits	(2)	0
Net change	31	26
Net balance at end of period	174	143
Plus reinsurance recoverables	87	101
Balance at end of period	261	244
Liability for guarantees related to income benefits
Liabilities for guarantees:
Liabilities for Guarantees on Long-Duration Contracts, Incurred Benefits	0	0
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Balance at beginning of period	43	67
Less reinsurance recoverables	40	64
Net balance at beginning of period	3	3
Paid guarantee benefits	0	0
Net change	0	0
Net balance at end of period	3	3
Plus reinsurance recoverables	25	40
Balance at end of period	28	43
Liability for guarantees related to accumulation and withdrawal benefits
Liabilities for guarantees:
Liabilities for Guarantees on Long-Duration Contracts, Incurred Benefits	12	11
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
Paid guarantee benefits	0	0
Net change	12	11
Net balance at end of period	46	34
Plus reinsurance recoverables	34	43
Balance at end of period	$ 80	$ 77