Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Total fair value within one year	$ 2,644
Total fair value after one but within five years	28,202
Total fair value after five but within ten years	6,535
Total fair value after ten years	24,976
Available-for-sale amount within one year	2,643
Available-for-sale yield within one year	1.21%
Available-for-sale amount after one but within five years	28,149
Available-for-sale yield after one but within five years	0.72%
Available-for-sale amount after five but within ten years	5,936
Available-for-sale yield after five but within ten years	2.73%
Available-for-sale amount after ten years	23,771
Available-for-sale yield after ten years	3.13%
Total fair value within one year	4
Total fair value after one but within five years	26
Total fair value after five but within ten years	15
Total fair value after ten years	1,932
Held-to-maturity amount within one year	4
Held-to-maturity yield within one year	5.37%
Held-to-maturity amount after one but within five years	25
Held-to-maturity yield after one but within five years	6.13%
Held-to-maturity amount after five but within ten years	14
Held-to-maturity yield after five but within ten years	5.49%
Held-to-maturity amount after ten years	1,698
Held-to-maturity yield after ten years	6.23%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount after one but within five years	295
Available-for-sale yield after one but within five years	2.77%
Available-for-sale amount after five but within ten years	2,675
Available-for-sale yield after five but within ten years	3.29%
Available-for-sale amount after ten years	766
Available-for-sale yield after ten years	3.44%
Held-to-maturity amount after one but within five years	8
Held-to-maturity yield after one but within five years	7.78%
Held-to-maturity amount after five but within ten years	1
Held-to-maturity yield after five but within ten years	7.87%
Held-to-maturity amount after ten years	1,203
Held-to-maturity yield after ten years	6.16%
U.S. Government agency issued or guaranteed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount after one but within five years	5
Available-for-sale yield after one but within five years	4.60%
Available-for-sale amount after five but within ten years	100
Available-for-sale yield after five but within ten years	1.97%
Available-for-sale amount after ten years	19,060
Available-for-sale yield after ten years	3.13%
Held-to-maturity amount after one but within five years	2
Held-to-maturity yield after one but within five years	7.58%
Held-to-maturity amount after five but within ten years	3
Held-to-maturity yield after five but within ten years	7.68%
Held-to-maturity amount after ten years	352
Held-to-maturity yield after ten years	6.49%
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount after one but within five years	40
Available-for-sale yield after one but within five years	4.16%
Available-for-sale amount after five but within ten years	376
Available-for-sale yield after five but within ten years	3.84%
Available-for-sale amount after ten years	359
Available-for-sale yield after ten years	3.84%
Held-to-maturity amount within one year	4
Held-to-maturity yield within one year	5.37%
Held-to-maturity amount after one but within five years	15
Held-to-maturity yield after one but within five years	5.11%
Held-to-maturity amount after five but within ten years	10
Held-to-maturity yield after five but within ten years	4.58%
Held-to-maturity amount after ten years	15
Held-to-maturity yield after ten years	4.96%
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount after one but within five years	1
Available-for-sale yield after one but within five years	1.46%
Available-for-sale amount after five but within ten years	11
Available-for-sale yield after five but within ten years	0.46%
Available-for-sale amount after ten years	671
Available-for-sale yield after ten years	3.04%
Held-to-maturity amount after ten years	128
Held-to-maturity yield after ten years	6.26%
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	2,002
Available-for-sale yield within one year	0.26%
Available-for-sale amount after one but within five years	22,945
Available-for-sale yield after one but within five years	0.45%
Available-for-sale amount after five but within ten years	2,774
Available-for-sale yield after five but within ten years	2.08%
Available-for-sale amount after ten years	2,891
Available-for-sale yield after ten years	2.99%
Corporate and other domestic [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	16
Available-for-sale yield within one year	0.62%
Available-for-sale amount after ten years	24
Available-for-sale yield after ten years	3.90%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	625
Available-for-sale yield within one year	4.27%
Available-for-sale amount after one but within five years	$ 4,863
Available-for-sale yield after one but within five years	1.83%