Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 1,207
Available-for-sale yield within one year	1.57%
Available-for-sale amount after one but within five years	17,987
Available-for-sale yield after one but within five years	1.14%
Available-for-sale amount after five but within ten years	6,022
Available-for-sale yield after five but within ten years	2.87%
Available-for-sale amount after ten years	22,325
Available-for-sale yield after ten years	2.86%
Total fair value within one year	1,210
Total fair value after one but within five years	18,027
Total fair value after five but within ten years	6,374
Total fair value after ten years	22,278
Held-to-maturity amount within one year	6
Held-to-maturity yield within one year	5.71%
Held-to-maturity amount after one but within five years	14
Held-to-maturity yield after one but within five years	5.18%
Held-to-maturity amount after five but within ten years	77
Held-to-maturity yield after five but within ten years	0.96%
Held-to-maturity amount after ten years	1,336
Held-to-maturity yield after ten years	5.61%
Total fair value within one year	6
Total fair value after one but within five years	16
Total fair value after five but within ten years	79
Total fair value after ten years	1,497
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	500
Available-for-sale yield within one year	0.32%
Available-for-sale amount after one but within five years	13,165
Available-for-sale yield after one but within five years	0.82%
Available-for-sale amount after five but within ten years	2,656
Available-for-sale yield after five but within ten years	2.45%
Available-for-sale amount after ten years	1,907
Available-for-sale yield after ten years	4.07%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	50
Available-for-sale yield within one year	0.38%
Available-for-sale amount after one but within five years	570
Available-for-sale yield after one but within five years	2.36%
Available-for-sale amount after five but within ten years	2,940
Available-for-sale yield after five but within ten years	3.20%
Available-for-sale amount after ten years	726
Available-for-sale yield after ten years	3.99%
Held-to-maturity amount within one year	1
Held-to-maturity yield within one year	8.00%
Held-to-maturity amount after one but within five years	1
Held-to-maturity yield after one but within five years	7.20%
Held-to-maturity amount after five but within ten years	2
Held-to-maturity yield after five but within ten years	8.02%
Held-to-maturity amount after ten years	889
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 within one year	0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	15
Available-for-sale yield after one but within five years	4.23%
Available-for-sale amount after five but within ten years	86
Available-for-sale yield after five but within ten years	2.28%
Available-for-sale amount after ten years	18,745
Available-for-sale yield after ten years	2.68%
Held-to-maturity amount within one year	0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	1
Held-to-maturity yield after one but within five years	7.62%
Held-to-maturity amount after five but within ten years	2
Held-to-maturity yield after five but within ten years	7.69%
Held-to-maturity amount after ten years	277
Held-to-maturity yield after ten years	6.51%
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	88
Available-for-sale yield after one but within five years	3.95%
Available-for-sale amount after five but within ten years	329
Available-for-sale yield after five but within ten years	3.54%
Available-for-sale amount after ten years	450
Available-for-sale yield after ten years	3.56%
Held-to-maturity amount within one year	5
Held-to-maturity yield within one year	4.99%
Held-to-maturity amount after one but within five years	12
Held-to-maturity yield after one but within five years	4.70%
Held-to-maturity amount after five but within ten years	6
Held-to-maturity yield after five but within ten years	3.96%
Held-to-maturity amount after ten years	8
Held-to-maturity yield after ten years	5.12%
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	1
Available-for-sale yield after one but within five years	2.02%
Available-for-sale amount after five but within ten years	11
Available-for-sale yield after five but within ten years	0.39%
Available-for-sale amount after ten years	497
Available-for-sale yield after ten years	2.62%
Held-to-maturity amount within one year	0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	0
Held-to-maturity yield after one but within five years	0.00%
Held-to-maturity amount after five but within ten years	0
Held-to-maturity yield after five but within ten years	0.00%
Held-to-maturity amount after ten years	21
Held-to-maturity yield after ten years	6.24%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	657
Available-for-sale yield within one year	2.60%
Available-for-sale amount after one but within five years	4,148
Available-for-sale yield after one but within five years	1.95%
Available-for-sale amount after five but within ten years
Available-for-sale yield after five but within ten years	0.00%
Available-for-sale amount after ten years	0
Available-for-sale yield after ten years	0.00%
Asset backed securities and other debt securities of VIE [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity amount within one year	0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	0
Held-to-maturity yield after one but within five years	0.00%
Held-to-maturity amount after five but within ten years	67
Held-to-maturity yield after five but within ten years	0.29%
Held-to-maturity amount after ten years	$ 141
Held-to-maturity yield after ten years	0.29%