Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 110,268	$ 113,450
Estimated Fair Value Assets	2,194	2,244
Estimated Fair Value Liabilities	3,141	3,129
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,067	1,964
Estimated Fair Value Assets	95	135
Estimated Fair Value Liabilities	124	75
Derivatives Designated as Hedging Instruments: | Fair Value Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	154	175
Estimated Fair Value Assets	36	44
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,913	1,789
Estimated Fair Value Assets	59	91
Estimated Fair Value Liabilities	124	75
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	27	27
Estimated Fair Value Assets	3	5
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	1,886	1,762
Estimated Fair Value Assets	56	86
Estimated Fair Value Liabilities	124	75
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	108,201	111,486
Estimated Fair Value Assets	2,099	2,109
Estimated Fair Value Liabilities	3,017	3,054
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	15,877	20,213
Estimated Fair Value Assets	660	922
Estimated Fair Value Liabilities	877	774
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate caps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,428	2,671
Estimated Fair Value Assets	25	7
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	282
Estimated Fair Value Assets	0	1
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	24,500	24,600
Estimated Fair Value Assets	136	133
Estimated Fair Value Liabilities	86	63
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,121	1,103
Estimated Fair Value Assets	53	69
Estimated Fair Value Liabilities	64	41
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	130	130
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — purchased
Derivatives, Fair Value [Line Items]
Gross Notional Amount	40	65
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	1	1
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Credit default swaps — written
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,906	1,878
Estimated Fair Value Assets	31	40
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	1,867	2,713
Estimated Fair Value Assets	0	15
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	47,581	47,066
Estimated Fair Value Assets	1,011	794
Estimated Fair Value Liabilities	1,568	1,664
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,575	8,998
Estimated Fair Value Assets	123	128
Estimated Fair Value Liabilities	421	430
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,894	1,767
Estimated Fair Value Assets	60	0
Estimated Fair Value Liabilities	$ 0	$ 79