Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,328
Distribution of Maturities: Amortized Cost, 1-5 Years	14,824
Distribution of Maturities: Amortized Cost, 5-10 Years	3,467
Distribution of Maturities: Amortized Cost, After 10 Years	767
Distribution of Maturities: Amortized Cost, Total	20,386
Distribution of Maturities: Fair Value, 1 Year or Less	1,387
Distribution of Maturities: Fair Value, 1-5 Years	15,561
Distribution of Maturities: Fair Value, 5-10 Years	3,523
Distribution of Maturities: Fair Value, After 10 Years	780
Distribution of Maturities: Fair Value, Total	21,251
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.15%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.98%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.33%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.83%	[1]
Available For Sale Securities Debt Maturities, Yield	2.87%	[1]
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	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	212
Distribution of Maturities: Fair Value, 1 Year or Less	11
Distribution of Maturities: Fair Value, 1-5 Years	211
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	222
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	130
Distribution of Maturities: Amortized Cost, 1-5 Years	1,381
Distribution of Maturities: Amortized Cost, 5-10 Years	340
Distribution of Maturities: Amortized Cost, After 10 Years	136
Distribution of Maturities: Amortized Cost, Total	1,987
Distribution of Maturities: Fair Value, 1 Year or Less	131
Distribution of Maturities: Fair Value, 1-5 Years	1,449
Distribution of Maturities: Fair Value, 5-10 Years	348
Distribution of Maturities: Fair Value, After 10 Years	141
Distribution of Maturities: Fair Value, Total	2,069
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	91
Distribution of Maturities: Amortized Cost, 1-5 Years	152
Distribution of Maturities: Amortized Cost, 5-10 Years	19
Distribution of Maturities: Amortized Cost, After 10 Years	48
Distribution of Maturities: Amortized Cost, Total	310
Distribution of Maturities: Fair Value, 1 Year or Less	93
Distribution of Maturities: Fair Value, 1-5 Years	161
Distribution of Maturities: Fair Value, 5-10 Years	20
Distribution of Maturities: Fair Value, After 10 Years	46
Distribution of Maturities: Fair Value, Total	320
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	980
Distribution of Maturities: Amortized Cost, 1-5 Years	12,875
Distribution of Maturities: Amortized Cost, 5-10 Years	3,006
Distribution of Maturities: Amortized Cost, After 10 Years	555
Distribution of Maturities: Amortized Cost, Total	17,416
Distribution of Maturities: Fair Value, 1 Year or Less	1,035
Distribution of Maturities: Fair Value, 1-5 Years	13,520
Distribution of Maturities: Fair Value, 5-10 Years	3,051
Distribution of Maturities: Fair Value, After 10 Years	563
Distribution of Maturities: Fair Value, Total	18,169
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	127
Distribution of Maturities: Amortized Cost, 5-10 Years	78
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	205
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	79
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	209
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	112
Distribution of Maturities: Amortized Cost, 1-5 Years	72
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	28
Distribution of Maturities: Amortized Cost, Total	214
Distribution of Maturities: Fair Value, 1 Year or Less	113
Distribution of Maturities: Fair Value, 1-5 Years	71
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	30
Distribution of Maturities: Fair Value, Total	216
Corporate And Other Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	4
Distribution of Maturities: Amortized Cost, 1-5 Years	16
Distribution of Maturities: Amortized Cost, 5-10 Years	22
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	19
Distribution of Maturities: Fair Value, 5-10 Years	23
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 46

[1] Average yields are based on amortized cost and presented on a fully taxable-equivalent basis.