Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,512
Distribution of Maturities: Amortized Cost, 1-5 Years	14,020
Distribution of Maturities: Amortized Cost, 5-10 Years	4,947
Distribution of Maturities: Amortized Cost, After 10 Years	1,824
Distribution of Maturities: Amortized Cost, Total	22,303
Distribution of Maturities: Fair Value, 1 Year or Less	1,578
Distribution of Maturities: Fair Value, 1-5 Years	14,674
Distribution of Maturities: Fair Value, 5-10 Years	4,993
Distribution of Maturities: Fair Value, After 10 Years	1,808
Distribution of Maturities: Fair Value, Total	23,053
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.35%
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.90%
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.18%
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.63%
Available For Sale Securities Debt Maturities, Yield	2.75%
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	11
Distribution of Maturities: Amortized Cost, 1-5 Years	201
Distribution of Maturities: Amortized Cost, 5-10 Years	593
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	805
Distribution of Maturities: Fair Value, 1 Year or Less	11
Distribution of Maturities: Fair Value, 1-5 Years	210
Distribution of Maturities: Fair Value, 5-10 Years	599
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	820
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	259
Distribution of Maturities: Amortized Cost, 1-5 Years	1,368
Distribution of Maturities: Amortized Cost, 5-10 Years	440
Distribution of Maturities: Amortized Cost, After 10 Years	144
Distribution of Maturities: Amortized Cost, Total	2,211
Distribution of Maturities: Fair Value, 1 Year or Less	260
Distribution of Maturities: Fair Value, 1-5 Years	1,430
Distribution of Maturities: Fair Value, 5-10 Years	446
Distribution of Maturities: Fair Value, After 10 Years	149
Distribution of Maturities: Fair Value, Total	2,285
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	80
Distribution of Maturities: Amortized Cost, 1-5 Years	136
Distribution of Maturities: Amortized Cost, 5-10 Years	18
Distribution of Maturities: Amortized Cost, After 10 Years	46
Distribution of Maturities: Amortized Cost, Total	280
Distribution of Maturities: Fair Value, 1 Year or Less	82
Distribution of Maturities: Fair Value, 1-5 Years	144
Distribution of Maturities: Fair Value, 5-10 Years	19
Distribution of Maturities: Fair Value, After 10 Years	45
Distribution of Maturities: Fair Value, Total	290
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,056
Distribution of Maturities: Amortized Cost, 1-5 Years	12,103
Distribution of Maturities: Amortized Cost, 5-10 Years	3,825
Distribution of Maturities: Amortized Cost, After 10 Years	1,634
Distribution of Maturities: Amortized Cost, Total	18,618
Distribution of Maturities: Fair Value, 1 Year or Less	1,118
Distribution of Maturities: Fair Value, 1-5 Years	12,670
Distribution of Maturities: Fair Value, 5-10 Years	3,853
Distribution of Maturities: Fair Value, After 10 Years	1,614
Distribution of Maturities: Fair Value, Total	19,255
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	145
Distribution of Maturities: Amortized Cost, 5-10 Years	50
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	195
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	150
Distribution of Maturities: Fair Value, 5-10 Years	52
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	202
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	102
Distribution of Maturities: Amortized Cost, 1-5 Years	50
Distribution of Maturities: Amortized Cost, 5-10 Years	1
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	153
Distribution of Maturities: Fair Value, 1 Year or Less	103
Distribution of Maturities: Fair Value, 1-5 Years	51
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	156
Corporate And Other Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	4
Distribution of Maturities: Amortized Cost, 1-5 Years	17
Distribution of Maturities: Amortized Cost, 5-10 Years	20
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	41
Distribution of Maturities: Fair Value, 1 Year or Less	4
Distribution of Maturities: Fair Value, 1-5 Years	19
Distribution of Maturities: Fair Value, 5-10 Years	22
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 45