Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,508
Distribution of Maturities: Amortized Cost, 1-5 Years	10,470
Distribution of Maturities: Amortized Cost, 5-10 Years	7,553
Distribution of Maturities: Amortized Cost, After 10 Years	4,384
Distribution of Maturities: Amortized Cost, Total	24,915
Distribution of Maturities: Fair Value, 1 Year or Less	2,649
Distribution of Maturities: Fair Value, 1-5 Years	10,679
Distribution of Maturities: Fair Value, 5-10 Years	7,569
Distribution of Maturities: Fair Value, After 10 Years	4,326
Distribution of Maturities: Fair Value, Total	25,223
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.47%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.46%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.89%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.09%	[1]
Available For Sale Securities Debt Maturities, Yield	2.70%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	1,007
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	1,007
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	1,011
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	1,011
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	75
Distribution of Maturities: Amortized Cost, 1-5 Years	239
Distribution of Maturities: Amortized Cost, 5-10 Years	531
Distribution of Maturities: Amortized Cost, After 10 Years	141
Distribution of Maturities: Amortized Cost, Total	986
Distribution of Maturities: Fair Value, 1 Year or Less	75
Distribution of Maturities: Fair Value, 1-5 Years	250
Distribution of Maturities: Fair Value, 5-10 Years	506
Distribution of Maturities: Fair Value, After 10 Years	139
Distribution of Maturities: Fair Value, Total	970
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	60
Distribution of Maturities: Amortized Cost, 1-5 Years	44
Distribution of Maturities: Amortized Cost, 5-10 Years	101
Distribution of Maturities: Amortized Cost, After 10 Years	23
Distribution of Maturities: Amortized Cost, Total	228
Distribution of Maturities: Fair Value, 1 Year or Less	60
Distribution of Maturities: Fair Value, 1-5 Years	47
Distribution of Maturities: Fair Value, 5-10 Years	105
Distribution of Maturities: Fair Value, After 10 Years	25
Distribution of Maturities: Fair Value, Total	237
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,349
Distribution of Maturities: Amortized Cost, 1-5 Years	9,020
Distribution of Maturities: Amortized Cost, 5-10 Years	6,919
Distribution of Maturities: Amortized Cost, After 10 Years	4,220
Distribution of Maturities: Amortized Cost, Total	22,508
Distribution of Maturities: Fair Value, 1 Year or Less	2,490
Distribution of Maturities: Fair Value, 1-5 Years	9,203
Distribution of Maturities: Fair Value, 5-10 Years	6,956
Distribution of Maturities: Fair Value, After 10 Years	4,162
Distribution of Maturities: Fair Value, Total	22,811
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	5
Distribution of Maturities: Amortized Cost, 1-5 Years	124
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	129
Distribution of Maturities: Fair Value, 1 Year or Less	5
Distribution of Maturities: Fair Value, 1-5 Years	127
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	132
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	14
Distribution of Maturities: Amortized Cost, 1-5 Years	3
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	19
Distribution of Maturities: Fair Value, 1 Year or Less	14
Distribution of Maturities: Fair Value, 1-5 Years	5
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	21
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	5
Distribution of Maturities: Amortized Cost, 1-5 Years	33
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	38
Distribution of Maturities: Fair Value, 1 Year or Less	5
Distribution of Maturities: Fair Value, 1-5 Years	36
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 41

[1] Average yields are based on amortized cost and presented on a FTE basis.