Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,204
Distribution of Maturities: Amortized Cost, 1-5 Years	17,570
Distribution of Maturities: Amortized Cost, 5-10 Years	1,953
Distribution of Maturities: Amortized Cost, After 10 Years	2,539
Distribution of Maturities: Amortized Cost, Total	23,266
Distribution of Maturities: Fair Value, 1 Year or Less	1,253
Distribution of Maturities: Fair Value, 1-5 Years	18,238
Distribution of Maturities: Fair Value, 5-10 Years	2,012
Distribution of Maturities: Fair Value, After 10 Years	2,567
Distribution of Maturities: Fair Value, Total	24,070
US Treasury Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	8
Distribution of Maturities: Amortized Cost, 1-5 Years	213
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	221
Distribution of Maturities: Fair Value, 1 Year or Less	8
Distribution of Maturities: Fair Value, 1-5 Years	223
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	231
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	99
Distribution of Maturities: Amortized Cost, 1-5 Years	1,391
Distribution of Maturities: Amortized Cost, 5-10 Years	91
Distribution of Maturities: Amortized Cost, After 10 Years	90
Distribution of Maturities: Amortized Cost, Total	1,671
Distribution of Maturities: Fair Value, 1 Year or Less	100
Distribution of Maturities: Fair Value, 1-5 Years	1,456
Distribution of Maturities: Fair Value, 5-10 Years	99
Distribution of Maturities: Fair Value, After 10 Years	92
Distribution of Maturities: Fair Value, Total	1,747
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	126
Distribution of Maturities: Amortized Cost, 1-5 Years	203
Distribution of Maturities: Amortized Cost, 5-10 Years	23
Distribution of Maturities: Amortized Cost, After 10 Years	54
Distribution of Maturities: Amortized Cost, Total	406
Distribution of Maturities: Fair Value, 1 Year or Less	129
Distribution of Maturities: Fair Value, 1-5 Years	217
Distribution of Maturities: Fair Value, 5-10 Years	23
Distribution of Maturities: Fair Value, After 10 Years	50
Distribution of Maturities: Fair Value, Total	419
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	797
Distribution of Maturities: Amortized Cost, 1-5 Years	15,376
Distribution of Maturities: Amortized Cost, 5-10 Years	1,750
Distribution of Maturities: Amortized Cost, After 10 Years	2,328
Distribution of Maturities: Amortized Cost, Total	20,251
Distribution of Maturities: Fair Value, 1 Year or Less	841
Distribution of Maturities: Fair Value, 1-5 Years	15,972
Distribution of Maturities: Fair Value, 5-10 Years	1,802
Distribution of Maturities: Fair Value, After 10 Years	2,352
Distribution of Maturities: Fair Value, Total	20,967
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	185
Distribution of Maturities: Amortized Cost, 5-10 Years	52
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	237
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	168
Distribution of Maturities: Fair Value, 5-10 Years	48
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	216
Collateralized Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	43
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	43
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	43
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	43
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	172
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	67
Distribution of Maturities: Amortized Cost, Total	394
Distribution of Maturities: Fair Value, 1 Year or Less	173
Distribution of Maturities: Fair Value, 1-5 Years	155
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	73
Distribution of Maturities: Fair Value, Total	401
Corporate And Other Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2
Distribution of Maturities: Amortized Cost, 1-5 Years	4
Distribution of Maturities: Amortized Cost, 5-10 Years	37
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	43
Distribution of Maturities: Fair Value, 1 Year or Less	2
Distribution of Maturities: Fair Value, 1-5 Years	4
Distribution of Maturities: Fair Value, 5-10 Years	40
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 46