Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 283,140	$ 275,916
Estimated Fair Value Assets	7,868	8,551
Estimated Fair Value Liabilities	4,135	4,339
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	55,617	57,020
Estimated Fair Value Assets	3,613	3,764
Estimated Fair Value Liabilities	2,210	2,023
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	5,869	8,212
Estimated Fair Value Assets	2,105	2,341
Estimated Fair Value Liabilities	37	58
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,485	3,843
Estimated Fair Value Assets	2,074	2,289
Estimated Fair Value Liabilities	1	3
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	665	1,116
Estimated Fair Value Assets	25	50
Estimated Fair Value Liabilities	0	18
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,719	3,253
Estimated Fair Value Assets	6	2
Estimated Fair Value Liabilities	36	37
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	40,276	39,068
Estimated Fair Value Assets	1,441	1,377
Estimated Fair Value Liabilities	1,918	1,797
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,490	3,584
Estimated Fair Value Assets	136	235
Estimated Fair Value Liabilities	17	4
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,073	3,332
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	221	128
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	33,713	32,152
Estimated Fair Value Assets	1,305	1,142
Estimated Fair Value Liabilities	1,680	1,665
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,472	9,740
Estimated Fair Value Assets	67	46
Estimated Fair Value Liabilities	255	168
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,413	332
Estimated Fair Value Assets	39	2
Estimated Fair Value Liabilities	18	5
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	8,059	9,408
Estimated Fair Value Assets	28	44
Estimated Fair Value Liabilities	237	163
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	227,523	218,896
Estimated Fair Value Assets	4,255	4,787
Estimated Fair Value Liabilities	1,925	2,316
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	53,532	60,485
Estimated Fair Value Assets	1,547	2,203
Estimated Fair Value Liabilities	239	576
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	175	217
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	37	42
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate floors
Derivatives, Fair Value [Line Items]
Gross Notional Amount	7,201	7,201
Estimated Fair Value Assets	56	92
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate caps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	45,375	53,079
Estimated Fair Value Assets	246	78
Estimated Fair Value Liabilities	1	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,715	4,366
Estimated Fair Value Assets	1	2
Estimated Fair Value Liabilities	5	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	23,998	12,009
Estimated Fair Value Assets	489	656
Estimated Fair Value Liabilities	0	11
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,048	1,048
Estimated Fair Value Assets	26	8
Estimated Fair Value Liabilities	0	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Synthetic GICs
Derivatives, Fair Value [Line Items]
Gross Notional Amount	17,184	11,318
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	12,333	9,902
Estimated Fair Value Assets	832	693
Estimated Fair Value Liabilities	435	506
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	14,885	12,238
Estimated Fair Value Assets	123	79
Estimated Fair Value Liabilities	264	190
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	869	846
Estimated Fair Value Assets	0	2
Estimated Fair Value Liabilities	6	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,270	3,123
Estimated Fair Value Assets	15	55
Estimated Fair Value Liabilities	5	6
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,948	2,020
Estimated Fair Value Assets	18	7
Estimated Fair Value Liabilities	40	43
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	11,836	11,375
Estimated Fair Value Assets	195	271
Estimated Fair Value Liabilities	4	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,272	4,005
Estimated Fair Value Assets	3	18
Estimated Fair Value Liabilities	4	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	23,207	19,886
Estimated Fair Value Assets	638	569
Estimated Fair Value Liabilities	667	690
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	4,661	4,661
Estimated Fair Value Assets	53	54
Estimated Fair Value Liabilities	200	199
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,014	1,117
Estimated Fair Value Assets	13	0
Estimated Fair Value Liabilities	$ 18	$ 41