Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 311,484	$ 275,916
Estimated Fair Value Assets	7,288	8,551
Estimated Fair Value Liabilities	4,517	4,339
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	56,120	57,020
Estimated Fair Value Assets	3,440	3,764
Estimated Fair Value Liabilities	2,432	2,023
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	5,956	8,212
Estimated Fair Value Assets	2,031	2,341
Estimated Fair Value Liabilities	76	58
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,450	3,843
Estimated Fair Value Assets	1,997	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	1,010	1,116
Estimated Fair Value Assets	34	50
Estimated Fair Value Liabilities	5	18
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,496	3,253
Estimated Fair Value Assets	0	2
Estimated Fair Value Liabilities	70	37
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	41,344	39,068
Estimated Fair Value Assets	1,362	1,377
Estimated Fair Value Liabilities	2,110	1,797
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,456	3,584
Estimated Fair Value Assets	113	235
Estimated Fair Value Liabilities	12	4
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,022	3,332
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	302	128
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	34,866	32,152
Estimated Fair Value Assets	1,249	1,142
Estimated Fair Value Liabilities	1,796	1,665
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	8,820	9,740
Estimated Fair Value Assets	47	46
Estimated Fair Value Liabilities	246	168
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,242	332
Estimated Fair Value Assets	20	2
Estimated Fair Value Liabilities	46	5
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	7,578	9,408
Estimated Fair Value Assets	27	44
Estimated Fair Value Liabilities	200	163
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	255,364	218,896
Estimated Fair Value Assets	3,848	4,787
Estimated Fair Value Liabilities	2,085	2,316
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	54,458	60,485
Estimated Fair Value Assets	1,368	2,203
Estimated Fair Value Liabilities	271	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	30	42
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate floors
Derivatives, Fair Value [Line Items]
Gross Notional Amount	12,701	7,201
Estimated Fair Value Assets	53	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	63,044	53,079
Estimated Fair Value Assets	258	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	2,274	4,366
Estimated Fair Value Assets	1	2
Estimated Fair Value Liabilities	4	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	23,125	12,009
Estimated Fair Value Assets	257	656
Estimated Fair Value Liabilities	1	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	0	8
Estimated Fair Value Liabilities	25	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Synthetic GICs
Derivatives, Fair Value [Line Items]
Gross Notional Amount	24,244	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	12,196	9,902
Estimated Fair Value Assets	914	693
Estimated Fair Value Liabilities	403	506
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	13,637	12,238
Estimated Fair Value Assets	52	79
Estimated Fair Value Liabilities	322	190
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	860	846
Estimated Fair Value Assets	0	2
Estimated Fair Value Liabilities	5	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,268	3,123
Estimated Fair Value Assets	16	55
Estimated Fair Value Liabilities	13	6
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,986	2,020
Estimated Fair Value Assets	27	7
Estimated Fair Value Liabilities	42	43
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	11,507	11,375
Estimated Fair Value Assets	222	271
Estimated Fair Value Liabilities	1	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,142	4,005
Estimated Fair Value Assets	8	18
Estimated Fair Value Liabilities	5	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	22,963	19,886
Estimated Fair Value Assets	613	569
Estimated Fair Value Liabilities	729	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	58	54
Estimated Fair Value Liabilities	206	199
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,075	1,117
Estimated Fair Value Assets	1	0
Estimated Fair Value Liabilities	$ 27	$ 41