Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions	Sep. 30, 2011
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,685
Distribution of Maturities: Amortized Cost, 1-5 Years	14,489
Distribution of Maturities: Amortized Cost, 5-10 Years	4,666
Distribution of Maturities: Amortized Cost, After 10 Years	3,077
Distribution of Maturities: Amortized Cost, Total	23,917
Distribution of Maturities: Fair Value, 1 Year or Less	1,730
Distribution of Maturities: Fair Value, 1-5 Years	15,007
Distribution of Maturities: Fair Value, 5-10 Years	4,862
Distribution of Maturities: Fair Value, After 10 Years	3,165
Distribution of Maturities: Fair Value, Total	24,764
US Treasury Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	9
Distribution of Maturities: Amortized Cost, 1-5 Years	213
Distribution of Maturities: Amortized Cost, 5-10 Years	152
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	374
Distribution of Maturities: Fair Value, 1 Year or Less	9
Distribution of Maturities: Fair Value, 1-5 Years	224
Distribution of Maturities: Fair Value, 5-10 Years	153
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	386
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	73
Distribution of Maturities: Amortized Cost, 1-5 Years	2,209
Distribution of Maturities: Amortized Cost, 5-10 Years	189
Distribution of Maturities: Amortized Cost, After 10 Years	56
Distribution of Maturities: Amortized Cost, Total	2,527
Distribution of Maturities: Fair Value, 1 Year or Less	74
Distribution of Maturities: Fair Value, 1-5 Years	2,309
Distribution of Maturities: Fair Value, 5-10 Years	204
Distribution of Maturities: Fair Value, After 10 Years	58
Distribution of Maturities: Fair Value, Total	2,645
US States and Political Subdivisions Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	136
Distribution of Maturities: Amortized Cost, 1-5 Years	245
Distribution of Maturities: Amortized Cost, 5-10 Years	26
Distribution of Maturities: Amortized Cost, After 10 Years	64
Distribution of Maturities: Amortized Cost, Total	471
Distribution of Maturities: Fair Value, 1 Year or Less	139
Distribution of Maturities: Fair Value, 1-5 Years	260
Distribution of Maturities: Fair Value, 5-10 Years	27
Distribution of Maturities: Fair Value, After 10 Years	64
Distribution of Maturities: Fair Value, Total	490
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,101
Distribution of Maturities: Amortized Cost, 1-5 Years	11,236
Distribution of Maturities: Amortized Cost, 5-10 Years	4,050
Distribution of Maturities: Amortized Cost, After 10 Years	2,915
Distribution of Maturities: Amortized Cost, Total	19,302
Distribution of Maturities: Fair Value, 1 Year or Less	1,138
Distribution of Maturities: Fair Value, 1-5 Years	11,644
Distribution of Maturities: Fair Value, 5-10 Years	4,246
Distribution of Maturities: Fair Value, After 10 Years	3,002
Distribution of Maturities: Fair Value, Total	20,030
Mortgage-backed Securities, Issued by Private Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	31
Distribution of Maturities: Amortized Cost, 1-5 Years	141
Distribution of Maturities: Amortized Cost, 5-10 Years	130
Distribution of Maturities: Amortized Cost, After 10 Years	17
Distribution of Maturities: Amortized Cost, Total	319
Distribution of Maturities: Fair Value, 1 Year or Less	28
Distribution of Maturities: Fair Value, 1-5 Years	129
Distribution of Maturities: Fair Value, 5-10 Years	114
Distribution of Maturities: Fair Value, After 10 Years	16
Distribution of Maturities: Fair Value, Total	287
Collateralized Debt Obligations
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	237
Distribution of Maturities: Amortized Cost, 5-10 Years	100
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	337
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	234
Distribution of Maturities: Fair Value, 5-10 Years	98
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	332
Asset-backed Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	328
Distribution of Maturities: Amortized Cost, 1-5 Years	204
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	534
Distribution of Maturities: Fair Value, 1 Year or Less	335
Distribution of Maturities: Fair Value, 1-5 Years	203
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	540
Corporate And Other Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	7
Distribution of Maturities: Amortized Cost, 1-5 Years	4
Distribution of Maturities: Amortized Cost, 5-10 Years	17
Distribution of Maturities: Amortized Cost, After 10 Years	25
Distribution of Maturities: Amortized Cost, Total	53
Distribution of Maturities: Fair Value, 1 Year or Less	7
Distribution of Maturities: Fair Value, 1-5 Years	4
Distribution of Maturities: Fair Value, 5-10 Years	18
Distribution of Maturities: Fair Value, After 10 Years	25
Distribution of Maturities: Fair Value, Total	$ 54

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