Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,787
Distribution of Maturities: Amortized Cost, 1-5 Years	6,947
Distribution of Maturities: Amortized Cost, 5-10 Years	9,094
Distribution of Maturities: Amortized Cost, After 10 Years	3,953
Distribution of Maturities: Amortized Cost, Total	21,781
Distribution of Maturities: Fair Value, 1 Year or Less	1,878
Distribution of Maturities: Fair Value, 1-5 Years	7,156
Distribution of Maturities: Fair Value, 5-10 Years	8,959
Distribution of Maturities: Fair Value, After 10 Years	3,707
Distribution of Maturities: Fair Value, Total	21,700
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.95%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.72%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.83%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.85%	[1]
Available For Sale Securities Debt Maturities, Yield	2.81%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1
Distribution of Maturities: Amortized Cost, 1-5 Years	645
Distribution of Maturities: Amortized Cost, 5-10 Years	688
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	1,334
Distribution of Maturities: Fair Value, 1 Year or Less	1
Distribution of Maturities: Fair Value, 1-5 Years	647
Distribution of Maturities: Fair Value, 5-10 Years	645
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	1,293
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	51
Distribution of Maturities: Amortized Cost, 1-5 Years	261
Distribution of Maturities: Amortized Cost, 5-10 Years	566
Distribution of Maturities: Amortized Cost, After 10 Years	150
Distribution of Maturities: Amortized Cost, Total	1,028
Distribution of Maturities: Fair Value, 1 Year or Less	51
Distribution of Maturities: Fair Value, 1-5 Years	271
Distribution of Maturities: Fair Value, 5-10 Years	518
Distribution of Maturities: Fair Value, After 10 Years	144
Distribution of Maturities: Fair Value, Total	984
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	102
Distribution of Maturities: Amortized Cost, 1-5 Years	66
Distribution of Maturities: Amortized Cost, 5-10 Years	21
Distribution of Maturities: Amortized Cost, After 10 Years	43
Distribution of Maturities: Amortized Cost, Total	232
Distribution of Maturities: Fair Value, 1 Year or Less	104
Distribution of Maturities: Fair Value, 1-5 Years	70
Distribution of Maturities: Fair Value, 5-10 Years	21
Distribution of Maturities: Fair Value, After 10 Years	42
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	1,575
Distribution of Maturities: Amortized Cost, 1-5 Years	5,780
Distribution of Maturities: Amortized Cost, 5-10 Years	7,800
Distribution of Maturities: Amortized Cost, After 10 Years	3,760
Distribution of Maturities: Amortized Cost, Total	18,915
Distribution of Maturities: Fair Value, 1 Year or Less	1,665
Distribution of Maturities: Fair Value, 1-5 Years	5,969
Distribution of Maturities: Fair Value, 5-10 Years	7,756
Distribution of Maturities: Fair Value, After 10 Years	3,521
Distribution of Maturities: Fair Value, Total	18,911
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	155
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	155
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	154
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	154
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	58
Distribution of Maturities: Amortized Cost, 1-5 Years	18
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	78
Distribution of Maturities: Fair Value, 1 Year or Less	57
Distribution of Maturities: Fair Value, 1-5 Years	20
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	79
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.