Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,935
Distribution of Maturities: Amortized Cost, 1-5 Years	7,484
Distribution of Maturities: Amortized Cost, 5-10 Years	8,760
Distribution of Maturities: Amortized Cost, After 10 Years	4,267
Distribution of Maturities: Amortized Cost, Total	22,446
Distribution of Maturities: Fair Value, 1 Year or Less	2,039
Distribution of Maturities: Fair Value, 1-5 Years	7,667
Distribution of Maturities: Fair Value, 5-10 Years	8,739
Distribution of Maturities: Fair Value, After 10 Years	4,091
Distribution of Maturities: Fair Value, Total	22,536
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.93%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.51%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.88%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.92%	[1]
Available For Sale Securities Debt Maturities, Yield	2.77%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1
Distribution of Maturities: Amortized Cost, 1-5 Years	893
Distribution of Maturities: Amortized Cost, 5-10 Years	688
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	1,582
Distribution of Maturities: Fair Value, 1 Year or Less	1
Distribution of Maturities: Fair Value, 1-5 Years	896
Distribution of Maturities: Fair Value, 5-10 Years	660
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	1,557
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	71
Distribution of Maturities: Amortized Cost, 1-5 Years	253
Distribution of Maturities: Amortized Cost, 5-10 Years	544
Distribution of Maturities: Amortized Cost, After 10 Years	147
Distribution of Maturities: Amortized Cost, Total	1,015
Distribution of Maturities: Fair Value, 1 Year or Less	71
Distribution of Maturities: Fair Value, 1-5 Years	264
Distribution of Maturities: Fair Value, 5-10 Years	509
Distribution of Maturities: Fair Value, After 10 Years	143
Distribution of Maturities: Fair Value, Total	987
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	97
Distribution of Maturities: Amortized Cost, 1-5 Years	57
Distribution of Maturities: Amortized Cost, 5-10 Years	94
Distribution of Maturities: Amortized Cost, After 10 Years	33
Distribution of Maturities: Amortized Cost, Total	281
Distribution of Maturities: Fair Value, 1 Year or Less	98
Distribution of Maturities: Fair Value, 1-5 Years	60
Distribution of Maturities: Fair Value, 5-10 Years	95
Distribution of Maturities: Fair Value, After 10 Years	34
Distribution of Maturities: Fair Value, Total	287
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,722
Distribution of Maturities: Amortized Cost, 1-5 Years	6,093
Distribution of Maturities: Amortized Cost, 5-10 Years	7,415
Distribution of Maturities: Amortized Cost, After 10 Years	4,087
Distribution of Maturities: Amortized Cost, Total	19,317
Distribution of Maturities: Fair Value, 1 Year or Less	1,825
Distribution of Maturities: Fair Value, 1-5 Years	6,252
Distribution of Maturities: Fair Value, 5-10 Years	7,456
Distribution of Maturities: Fair Value, After 10 Years	3,914
Distribution of Maturities: Fair Value, Total	19,447
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	147
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	147
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	149
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	149
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	44
Distribution of Maturities: Amortized Cost, 1-5 Years	19
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	65
Distribution of Maturities: Fair Value, 1 Year or Less	44
Distribution of Maturities: Fair Value, 1-5 Years	21
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	67
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	22
Distribution of Maturities: Amortized Cost, 5-10 Years	17
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	39
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	25
Distribution of Maturities: Fair Value, 5-10 Years	17
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 42

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