Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 121,890	$ 131,396
Estimated Fair Value Assets	2,465	3,603
Estimated Fair Value Liabilities	3,136	3,883
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,761	1,775
Estimated Fair Value Assets	160	234
Estimated Fair Value Liabilities	48	10
Derivatives Designated as Hedging Instruments: | Fair Value Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	182	310
Estimated Fair Value Assets	44	41
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,579	1,465
Estimated Fair Value Assets	116	193
Estimated Fair Value Liabilities	48	10
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	45	45
Estimated Fair Value Assets	8	7
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,534	1,420
Estimated Fair Value Assets	108	186
Estimated Fair Value Liabilities	48	10
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	120,129	129,621
Estimated Fair Value Assets	2,305	3,369
Estimated Fair Value Liabilities	3,088	3,873
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	20,227	28,175
Estimated Fair Value Assets	994	1,928
Estimated Fair Value Liabilities	786	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	5,032	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	0	9
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	24,600	15,520
Estimated Fair Value Assets	183	136
Estimated Fair Value Liabilities	28	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Total Return Swap [Member]
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: | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	959	1,250
Estimated Fair Value Assets	80	153
Estimated Fair Value Liabilities	25	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	166	158
Estimated Fair Value Assets	1	9
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	34	34
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,879	1,891
Estimated Fair Value Assets	37	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	2,535	8,037
Estimated Fair Value Assets	0	38
Estimated Fair Value Liabilities	11	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	47,727	37,501
Estimated Fair Value Assets	827	897
Estimated Fair Value Liabilities	1,568	934
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	14,894	14,894
Estimated Fair Value Assets	176	140
Estimated Fair Value Liabilities	607	517
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,794	2,855
Estimated Fair Value Assets	0	1
Estimated Fair Value Liabilities	$ 63	$ 117