Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 729
Available-for-sale yield within one year	1.90%
Available-for-sale amount after one but within five years	14,525
Available-for-sale yield after one but within five years	1.33%
Available-for-sale amount after five but within ten years	4,161
Available-for-sale yield after five but within ten years	3.42%
Available-for-sale amount after ten years	9,658
Available-for-sale yield after ten years	3.18%
Total fair value within one year	729
Total fair value after one but within five years	14,595
Total fair value after five but within ten years	4,455
Total fair value after ten years	9,857
Held-to-maturity amount within one year	5
Held-to-maturity yield within one year	5.52%
Held-to-maturity amount after one but within five years	178
Held-to-maturity yield after one but within five years	1.11%
Held-to-maturity after five but within ten years	464
Held-to-maturity yield after five but within ten years	2.84%
Held-to-maturity after ten years	11,398
Held-to-maturity yield after ten years	2.73%
Total fair value within one year	5
Total fair value after one but within five years	240
Total fair value after five but within ten years	467
Total fair value after ten years	11,509
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	275
Available-for-sale yield within one year	0.35%
Available-for-sale amount after one but within five years	9,446
Available-for-sale yield after one but within five years	0.84%
Available-for-sale amount after five but within ten years	1,514
Available-for-sale yield after five but within ten years	3.67%
Available-for-sale amount after ten years	1,853
Available-for-sale yield after ten years	3.73%
US Government sponsored enterprises [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	1,074
Available-for-sale yield after one but within five years	3.08%
Available-for-sale amount after five but within ten years	2,316
Available-for-sale yield after five but within ten years	3.26%
Available-for-sale amount after ten years	1,122
Available-for-sale yield after ten years	3.31%
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	92
Held-to-maturity yield after one but within five years	1.38%
Held-to-maturity after five but within ten years	407
Held-to-maturity yield after five but within ten years	2.86%
Held-to-maturity after ten years	3,903
Held-to-maturity yield after ten years	3.22%
US 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	11
Available-for-sale yield after one but within five years	4.22%
Available-for-sale amount after five but within ten years	47
Available-for-sale yield after five but within ten years	3.89%
Available-for-sale amount after ten years	6,119
Available-for-sale yield after ten years	2.97%
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	7.63%
Held-to-maturity after five but within ten years	52
Held-to-maturity yield after five but within ten years	2.56%
Held-to-maturity after ten years	7,478
Held-to-maturity yield after ten years	2.47%
Obligations of U.S. states and political subdivision [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	121
Available-for-sale yield after one but within five years	3.72%
Available-for-sale amount after five but within ten years	284
Available-for-sale yield after five but within ten years	3.27%
Available-for-sale amount after ten years	297
Available-for-sale yield after ten years	3.45%
Held-to-maturity amount within one year	5
Held-to-maturity yield within one year	5.52%
Held-to-maturity amount after one but within five years	9
Held-to-maturity yield after one but within five years	3.97%
Held-to-maturity after five but within ten years	5
Held-to-maturity yield after five but within ten years	4.00%
Held-to-maturity after ten years	5
Held-to-maturity yield after ten years	4.98%
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	0
Available-for-sale yield after one but within five years	0.00%
Available-for-sale amount after five but within ten years	0
Available-for-sale yield after five but within ten years	0.00%
Available-for-sale amount after ten years	267
Available-for-sale yield after ten years	3.48%
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 after five but within ten years	0
Held-to-maturity yield after five but within ten years	0.00%
Held-to-maturity after ten years	12
Held-to-maturity yield after ten years	6.41%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	454
Available-for-sale yield within one year	2.84%
Available-for-sale amount after one but within five years	3,873
Available-for-sale yield after one but within five years	1.95%
Available-for-sale amount after five but within ten years	0
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 issued by consolidated 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	76
Held-to-maturity yield after one but within five years	0.39%
Held-to-maturity after five but within ten years	0
Held-to-maturity yield after five but within ten years	0.00%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years	0.00%