Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,264
Distribution of Maturities: Amortized Cost, 1-5 Years	16,346
Distribution of Maturities: Amortized Cost, 5-10 Years	2,071
Distribution of Maturities: Amortized Cost, After 10 Years	509
Distribution of Maturities: Amortized Cost, Total	20,190
Distribution of Maturities: Fair Value, 1 Year or Less	1,318
Distribution of Maturities: Fair Value, 1-5 Years	17,080
Distribution of Maturities: Fair Value, 5-10 Years	2,167
Distribution of Maturities: Fair Value, After 10 Years	525
Distribution of Maturities: Fair Value, Total	21,090
US Treasury Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	12
Distribution of Maturities: Amortized Cost, 1-5 Years	202
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	214
Distribution of Maturities: Fair Value, 1 Year or Less	12
Distribution of Maturities: Fair Value, 1-5 Years	212
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	224
US Government Agencies Debt Securities
Distribution of Maturities: Amortized Cost, 1 Year or Less	117
Distribution of Maturities: Amortized Cost, 1-5 Years	1,372
Distribution of Maturities: Amortized Cost, 5-10 Years	95
Distribution of Maturities: Amortized Cost, After 10 Years	114
Distribution of Maturities: Amortized Cost, Total	1,698
Distribution of Maturities: Fair Value, 1 Year or Less	118
Distribution of Maturities: Fair Value, 1-5 Years	1,441
Distribution of Maturities: Fair Value, 5-10 Years	105
Distribution of Maturities: Fair Value, After 10 Years	119
Distribution of Maturities: Fair Value, Total	1,783
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	108
Distribution of Maturities: Amortized Cost, 1-5 Years	178
Distribution of Maturities: Amortized Cost, 5-10 Years	21
Distribution of Maturities: Amortized Cost, After 10 Years	52
Distribution of Maturities: Amortized Cost, Total	359
Distribution of Maturities: Fair Value, 1 Year or Less	111
Distribution of Maturities: Fair Value, 1-5 Years	191
Distribution of Maturities: Fair Value, 5-10 Years	21
Distribution of Maturities: Fair Value, After 10 Years	49
Distribution of Maturities: Fair Value, Total	372
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Distribution of Maturities: Amortized Cost, 1 Year or Less	901
Distribution of Maturities: Amortized Cost, 1-5 Years	14,304
Distribution of Maturities: Amortized Cost, 5-10 Years	1,827
Distribution of Maturities: Amortized Cost, After 10 Years	276
Distribution of Maturities: Amortized Cost, Total	17,308
Distribution of Maturities: Fair Value, 1 Year or Less	951
Distribution of Maturities: Fair Value, 1-5 Years	14,957
Distribution of Maturities: Fair Value, 5-10 Years	1,916
Distribution of Maturities: Fair Value, After 10 Years	286
Distribution of Maturities: Fair Value, Total	18,110
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	136
Distribution of Maturities: Amortized Cost, 5-10 Years	89
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	225
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	125
Distribution of Maturities: Fair Value, 5-10 Years	83
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	208
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	123
Distribution of Maturities: Amortized Cost, 1-5 Years	152
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	67
Distribution of Maturities: Amortized Cost, Total	344
Distribution of Maturities: Fair Value, 1 Year or Less	123
Distribution of Maturities: Fair Value, 1-5 Years	152
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	71
Distribution of Maturities: Fair Value, Total	348
Corporate And Other Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	3
Distribution of Maturities: Amortized Cost, 1-5 Years	2
Distribution of Maturities: Amortized Cost, 5-10 Years	37
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	3
Distribution of Maturities: Fair Value, 1-5 Years	2
Distribution of Maturities: Fair Value, 5-10 Years	40
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 45