Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,377
Distribution of Maturities: Amortized Cost, 1-5 Years	16,483
Distribution of Maturities: Amortized Cost, 5-10 Years	1,233
Distribution of Maturities: Amortized Cost, After 10 Years	396
Distribution of Maturities: Amortized Cost, Total	19,489
Distribution of Maturities: Fair Value, 1 Year or Less	1,441
Distribution of Maturities: Fair Value, 1-5 Years	17,332
Distribution of Maturities: Fair Value, 5-10 Years	1,305
Distribution of Maturities: Fair Value, After 10 Years	413
Distribution of Maturities: Fair Value, Total	20,491
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.47%
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.95%
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	3.68%
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.38%
Available For Sale Securities Debt Maturities, Yield	3.03%
US Treasury Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	11
Distribution of Maturities: Amortized Cost, 1-5 Years	202
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	213
Distribution of Maturities: Fair Value, 1 Year or Less	11
Distribution of Maturities: Fair Value, 1-5 Years	213
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	224
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	153
Distribution of Maturities: Amortized Cost, 1-5 Years	1,349
Distribution of Maturities: Amortized Cost, 5-10 Years	84
Distribution of Maturities: Amortized Cost, After 10 Years	127
Distribution of Maturities: Amortized Cost, Total	1,713
Distribution of Maturities: Fair Value, 1 Year or Less	155
Distribution of Maturities: Fair Value, 1-5 Years	1,420
Distribution of Maturities: Fair Value, 5-10 Years	95
Distribution of Maturities: Fair Value, After 10 Years	132
Distribution of Maturities: Fair Value, Total	1,802
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	103
Distribution of Maturities: Amortized Cost, 1-5 Years	172
Distribution of Maturities: Amortized Cost, 5-10 Years	19
Distribution of Maturities: Amortized Cost, After 10 Years	49
Distribution of Maturities: Amortized Cost, Total	343
Distribution of Maturities: Fair Value, 1 Year or Less	105
Distribution of Maturities: Fair Value, 1-5 Years	184
Distribution of Maturities: Fair Value, 5-10 Years	19
Distribution of Maturities: Fair Value, After 10 Years	47
Distribution of Maturities: Fair Value, Total	355
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	947
Distribution of Maturities: Amortized Cost, 1-5 Years	14,539
Distribution of Maturities: Amortized Cost, 5-10 Years	1,027
Distribution of Maturities: Amortized Cost, After 10 Years	192
Distribution of Maturities: Amortized Cost, Total	16,705
Distribution of Maturities: Fair Value, 1 Year or Less	1,006
Distribution of Maturities: Fair Value, 1-5 Years	15,294
Distribution of Maturities: Fair Value, 5-10 Years	1,082
Distribution of Maturities: Fair Value, After 10 Years	204
Distribution of Maturities: Fair Value, Total	17,586
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	129
Distribution of Maturities: Amortized Cost, 5-10 Years	84
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	213
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	130
Distribution of Maturities: Fair Value, 5-10 Years	87
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	217
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	159
Distribution of Maturities: Amortized Cost, 1-5 Years	71
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	28
Distribution of Maturities: Amortized Cost, Total	260
Distribution of Maturities: Fair Value, 1 Year or Less	160
Distribution of Maturities: Fair Value, 1-5 Years	69
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	30
Distribution of Maturities: Fair Value, Total	261
Corporate And Other Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	4
Distribution of Maturities: Amortized Cost, 1-5 Years	21
Distribution of Maturities: Amortized Cost, 5-10 Years	17
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	42
Distribution of Maturities: Fair Value, 1 Year or Less	4
Distribution of Maturities: Fair Value, 1-5 Years	22
Distribution of Maturities: Fair Value, 5-10 Years	20
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 46