Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 262,217	$ 275,916
Estimated Fair Value Assets	7,924	8,551
Estimated Fair Value Liabilities	4,075	4,339
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	56,148	57,020
Estimated Fair Value Assets	3,662	3,764
Estimated Fair Value Liabilities	2,320	2,023
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	7,144	8,212
Estimated Fair Value Assets	2,273	2,341
Estimated Fair Value Liabilities	42	58
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,521	3,843
Estimated Fair Value Assets	2,111	2,289
Estimated Fair Value Liabilities	2	3
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,146	1,116
Estimated Fair Value Assets	65	50
Estimated Fair Value Liabilities	36	18
Derivatives Designated as Hedging Instruments: | Fair Value Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,477	3,253
Estimated Fair Value Assets	97	2
Estimated Fair Value Liabilities	4	37
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	39,959	39,068
Estimated Fair Value Assets	1,368	1,377
Estimated Fair Value Liabilities	2,010	1,797
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,580	3,584
Estimated Fair Value Assets	158	235
Estimated Fair Value Liabilities	14	4
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,143	3,332
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	218	128
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges [Member] | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	33,236	32,152
Estimated Fair Value Assets	1,210	1,142
Estimated Fair Value Liabilities	1,778	1,665
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member]
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,045	9,740
Estimated Fair Value Assets	21	46
Estimated Fair Value Liabilities	268	168
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	458	332
Estimated Fair Value Assets	2	2
Estimated Fair Value Liabilities	5	5
Derivatives Designated as Hedging Instruments: | Foreign Operations Hedges [Member] | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	8,587	9,408
Estimated Fair Value Assets	19	44
Estimated Fair Value Liabilities	263	163
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	206,069	218,896
Estimated Fair Value Assets	4,262	4,787
Estimated Fair Value Liabilities	1,755	2,316
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	52,610	60,485
Estimated Fair Value Assets	1,597	2,203
Estimated Fair Value Liabilities	279	576
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	217	217
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	51	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	66	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	46,020	53,079
Estimated Fair Value Assets	221	78
Estimated Fair Value Liabilities	2	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,991	4,366
Estimated Fair Value Assets	2	2
Estimated Fair Value Liabilities	3	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	14,816	12,009
Estimated Fair Value Assets	575	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	14	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	11,720	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	10,773	9,902
Estimated Fair Value Assets	580	693
Estimated Fair Value Liabilities	461	506
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	13,803	12,238
Estimated Fair Value Assets	295	79
Estimated Fair Value Liabilities	67	190
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	900	846
Estimated Fair Value Assets	0	2
Estimated Fair Value Liabilities	8	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Currency options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,391	3,123
Estimated Fair Value Assets	9	55
Estimated Fair Value Liabilities	0	6
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,888	2,020
Estimated Fair Value Assets	5	7
Estimated Fair Value Liabilities	41	43
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	11,421	11,375
Estimated Fair Value Assets	210	271
Estimated Fair Value Liabilities	2	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity futures
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,088	4,005
Estimated Fair Value Assets	7	18
Estimated Fair Value Liabilities	19	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	19,509	19,886
Estimated Fair Value Assets	594	569
Estimated Fair Value Liabilities	603	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	52	54
Estimated Fair Value Liabilities	194	199
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,012	1,117
Estimated Fair Value Assets	35	0
Estimated Fair Value Liabilities	$ 0	$ 41