Derivatives (Primary Risks) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Notional Amount	$ 103,441	$ 113,450
Estimated Fair Value Assets	1,846	2,244
Estimated Fair Value Liabilities	3,304	3,129
Derivatives Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,283	1,964
Estimated Fair Value Assets	108	135
Estimated Fair Value Liabilities	65	75
Derivatives Designated as Hedging Instruments: | Fair Value Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	0	175
Estimated Fair Value Assets	0	44
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,283	1,789
Estimated Fair Value Assets	108	91
Estimated Fair Value Liabilities	65	75
Derivatives Designated as Hedging Instruments: | Cash Flow Hedges | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	0	27
Estimated Fair Value Assets	0	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	2,283	1,762
Estimated Fair Value Assets	108	86
Estimated Fair Value Liabilities	65	75
Derivatives Not Designated or Not Qualifying as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Gross Notional Amount	101,158	111,486
Estimated Fair Value Assets	1,738	2,109
Estimated Fair Value Liabilities	3,239	3,054
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	15,237	20,213
Estimated Fair Value Assets	546	922
Estimated Fair Value Liabilities	940	774
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Interest rate caps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	3,350	2,671
Estimated Fair Value Assets	32	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	54	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	13,819	24,600
Estimated Fair Value Assets	51	133
Estimated Fair Value Liabilities	104	63
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	1,127	1,103
Estimated Fair Value Assets	67	69
Estimated Fair Value Liabilities	28	41
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Gross Notional Amount	125	130
Estimated Fair Value Assets	1	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	86	65
Estimated Fair Value Assets	3	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	1,872	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	2,215	2,713
Estimated Fair Value Assets	1	15
Estimated Fair Value Liabilities	1	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity index options
Derivatives, Fair Value [Line Items]
Gross Notional Amount	51,044	47,066
Estimated Fair Value Assets	862	794
Estimated Fair Value Liabilities	1,657	1,664
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity variance swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	9,713	8,998
Estimated Fair Value Assets	143	128
Estimated Fair Value Liabilities	445	430
Derivatives Not Designated or Not Qualifying as Hedging Instruments: | Equity total return swaps
Derivatives, Fair Value [Line Items]
Gross Notional Amount	2,516	1,767
Estimated Fair Value Assets	1	0
Estimated Fair Value Liabilities	$ 63	$ 79