Derivatives (Primary Risks) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Notional Amount	$ 63,045	$ 56,193
Estimated Fair Value Assets	1,703	2,370
Estimated Fair Value Liabilities	765	970
Derivatives Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	2,168	2,252
Estimated Fair Value Assets	101	224
Estimated Fair Value Liabilities	43	37
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	709	660
Estimated Fair Value Assets	15	28
Estimated Fair Value Liabilities	27	23
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	587	538
Estimated Fair Value Assets	15	28
Estimated Fair Value Liabilities	5	9
Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedges [Member] | Foreign currency swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	122	122
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	22	14
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,459	1,592
Estimated Fair Value Assets	86	196
Estimated Fair Value Liabilities	16	14
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	598	658
Estimated Fair Value Assets	29	99
Estimated Fair Value Liabilities	12	0
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Interest rate forwards [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	310	410
Estimated Fair Value Assets	27	81
Estimated Fair Value Liabilities	0	0
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Foreign currency swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	546	524
Estimated Fair Value Assets	30	16
Estimated Fair Value Liabilities	4	14
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedges [Member] | Credit forwards [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	5	0
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	60,877	53,941
Estimated Fair Value Assets	1,602	2,146
Estimated Fair Value Liabilities	722	933
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	22,660	16,869
Estimated Fair Value Assets	902	1,254
Estimated Fair Value Liabilities	431	513
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate floors [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	17,604	15,136
Estimated Fair Value Assets	162	318
Estimated Fair Value Liabilities	156	274
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate caps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	8,001	9,031
Estimated Fair Value Assets	36	11
Estimated Fair Value Liabilities	0	0
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Interest rate futures [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,743	2,771
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	1	7
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Foreign currency swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	834	811
Estimated Fair Value Assets	58	60
Estimated Fair Value Liabilities	25	35
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Foreign currency forwards [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	43	139
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	0	4
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Credit default swaps - purchased [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	198	162
Estimated Fair Value Assets	0	0
Estimated Fair Value Liabilities	2	2
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Credit default swaps - written [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	2,393	2,456
Estimated Fair Value Assets	23	23
Estimated Fair Value Liabilities	0	1
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity futures [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	971	1,075
Estimated Fair Value Assets	3	0
Estimated Fair Value Liabilities	0	27
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Equity Options [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	3,431	2,845
Estimated Fair Value Assets	411	469
Estimated Fair Value Liabilities	14	1
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Variance swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	2,562	2,346
Estimated Fair Value Assets	1	11
Estimated Fair Value Liabilities	85	62
Derivatives Not Designated or Not Qualifying as Hedging Instruments [Member] | Total rate of return swaps [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	437	300
Estimated Fair Value Assets	6	0
Estimated Fair Value Liabilities	$ 8	$ 7