Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 275,916	$ 286,643
Estimated Fair Value Assets	8,551	12,139
Estimated Fair Value Liabilities	4,339	6,921
Derivatives Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	57,020	50,351
Estimated Fair Value Assets	3,764	4,652
Estimated Fair Value Liabilities	2,023	2,792
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	8,212	7,327
Estimated Fair Value Assets	2,341	2,255
Estimated Fair Value Liabilities	58	284
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,843	5,021
Estimated Fair Value Assets	2,289	2,221
Estimated Fair Value Liabilities	3	6
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,116	1,221
Estimated Fair Value Assets	50	34
Estimated Fair Value Liabilities	18	224
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,253	1,085
Estimated Fair Value Assets	2	0
Estimated Fair Value Liabilities	37	54
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	39,068	32,752
Estimated Fair Value Assets	1,377	2,202
Estimated Fair Value Liabilities	1,797	2,458
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,584	2,040
Estimated Fair Value Assets	235	325
Estimated Fair Value Liabilities	4	34
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,332	4,032
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	128	370
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	32,152	26,680
Estimated Fair Value Assets	1,142	1,877
Estimated Fair Value Liabilities	1,665	2,054
Derivatives Designated as Hedging Instruments [Member] | Foreign Operations Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,740	10,272
Estimated Fair Value Assets	46	195
Estimated Fair Value Liabilities	168	50
Derivatives Designated as Hedging Instruments [Member] | Foreign Operations Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	332	1,394
Estimated Fair Value Assets	2	47
Estimated Fair Value Liabilities	5	5
Derivatives Designated as Hedging Instruments [Member] | Foreign Operations Hedges [Member] | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,408	8,878
Estimated Fair Value Assets	44	148
Estimated Fair Value Liabilities	163	45
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	218,896	236,292
Estimated Fair Value Assets	4,787	7,487
Estimated Fair Value Liabilities	2,316	4,129
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	60,485	53,349
Estimated Fair Value Assets	2,203	4,089
Estimated Fair Value Liabilities	576	1,641
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	217	613
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	42	25
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate floors
Derivatives, Fair Value [Line Items]
Gross Notional Amount	7,201	12,101
Estimated Fair Value Assets	92	181
Estimated Fair Value Liabilities	0	7
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate caps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	53,079	78,358
Estimated Fair Value Assets	78	112
Estimated Fair Value Liabilities	2	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	4,366	4,793
Estimated Fair Value Assets	2	3
Estimated Fair Value Liabilities	4	12
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	12,009	5,334
Estimated Fair Value Assets	656	628
Estimated Fair Value Liabilities	11	1
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Synthetic GICs
Derivatives, Fair Value [Line Items]
Gross Notional Amount	11,318	5,566
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,902	11,651
Estimated Fair Value Assets	693	1,445
Estimated Fair Value Liabilities	506	462
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	12,238	15,422
Estimated Fair Value Assets	79	117
Estimated Fair Value Liabilities	190	977
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Currency futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	846	915
Estimated Fair Value Assets	2	0
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,123	3,615
Estimated Fair Value Assets	55	195
Estimated Fair Value Liabilities	6	17
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,020	2,001
Estimated Fair Value Assets	7	14
Estimated Fair Value Liabilities	43	40
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	11,375	10,732
Estimated Fair Value Assets	271	161
Estimated Fair Value Liabilities	0	9
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	4,005	4,457
Estimated Fair Value Assets	18	30
Estimated Fair Value Liabilities	4	3
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	19,886	16,527
Estimated Fair Value Assets	569	426
Estimated Fair Value Liabilities	690	523
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	4,661	8,263
Estimated Fair Value Assets	54	83
Estimated Fair Value Liabilities	199	240
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Total rate of return swaps [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,048	1,549
Estimated Fair Value Assets	8	2
Estimated Fair Value Liabilities	2	127
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity Total Return Swaps [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,117	1,046
Estimated Fair Value Assets	0	1
Estimated Fair Value Liabilities	$ 41	$ 43