Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 113,450	$ 131,396
Estimated Fair Value Assets	2,244	3,603
Estimated Fair Value Liabilities	3,129	3,883
Derivatives Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,964	1,775
Estimated Fair Value Assets	135	234
Estimated Fair Value Liabilities	75	10
Derivatives Designated as Hedging Instruments | Fair Value Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	175	310
Estimated Fair Value Assets	44	41
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,789	1,465
Estimated Fair Value Assets	91	193
Estimated Fair Value Liabilities	75	10
Derivatives Designated as Hedging Instruments | Cash Flow Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	27	45
Estimated Fair Value Assets	5	7
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments | Cash Flow Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,762	1,420
Estimated Fair Value Assets	86	186
Estimated Fair Value Liabilities	75	10
Derivatives Not Designated or Not Qualifying as Hedging Instruments
Derivatives, Fair Value [Line Items]
Gross Notional Amount	111,486	129,621
Estimated Fair Value Assets	2,109	3,369
Estimated Fair Value Liabilities	3,054	3,873
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	20,213	28,175
Estimated Fair Value Assets	922	1,928
Estimated Fair Value Liabilities	774	1,688
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest rate floors
Derivatives, Fair Value [Line Items]
Gross Notional Amount	0	2,100
Estimated Fair Value Assets	0	5
Estimated Fair Value Liabilities	0	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest rate caps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,671	12,042
Estimated Fair Value Assets	7	25
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest rate futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	282	1,288
Estimated Fair Value Assets	1	9
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest Rate Swaption [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	24,600	15,520
Estimated Fair Value Assets	133	136
Estimated Fair Value Liabilities	63	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,103	1,250
Estimated Fair Value Assets	69	153
Estimated Fair Value Liabilities	41	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	130	158
Estimated Fair Value Assets	0	9
Estimated Fair Value Liabilities	2	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	65	34
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	1	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,878	1,891
Estimated Fair Value Assets	40	28
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Equity futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,713	8,037
Estimated Fair Value Assets	15	38
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	47,066	37,501
Estimated Fair Value Assets	794	897
Estimated Fair Value Liabilities	1,664	934
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	8,998	14,894
Estimated Fair Value Assets	128	140
Estimated Fair Value Liabilities	430	517
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Interest rate total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	0	3,876
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	611
Derivatives Not Designated or Not Qualifying as Hedging Instruments | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,767	2,855
Estimated Fair Value Assets	0	1
Estimated Fair Value Liabilities	$ 79	$ 117