Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 1,860
Available-for-sale yield within one year	1.26%
Available-for-sale amount after one but within five years	26,975
Available-for-sale yield after one but within five years	0.97%
Available-for-sale amount after five but within ten years	5,720
Available-for-sale yield after five but within ten years	3.09%
Available-for-sale amount after ten years	18,687
Available-for-sale yield after ten years	2.99%
Total fair value within one year	1,863
Total fair value after one but within five years	27,009
Total fair value after five but within ten years	6,023
Total fair value after ten years	18,530
Held-to-maturity amount within one year	6
Held-to-maturity yield within one year	5.63%
Held-to-maturity amount after one but within five years	11
Held-to-maturity yield after one but within five years	4.92%
Held-to-maturity amount after five but within ten years	73
Held-to-maturity yield after five but within ten years	1.02%
Held-to-maturity amount after ten years	1,195
Held-to-maturity yield after ten years	5.64%
Total fair value within one year	5
Total fair value after one but within five years	12
Total fair value after five but within ten years	76
Total fair value after ten years	1,340
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	1,224
Available-for-sale yield within one year	0.34%
Available-for-sale amount after one but within five years	21,867
Available-for-sale yield after one but within five years	0.71%
Available-for-sale amount after five but within ten years	2,723
Available-for-sale yield after five but within ten years	2.82%
Available-for-sale amount after ten years	1,785
Available-for-sale yield after ten years	3.98%
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	980
Available-for-sale yield after one but within five years	2.42%
Available-for-sale amount after five but within ten years	2,597
Available-for-sale yield after five but within ten years	3.37%
Available-for-sale amount after ten years	734
Available-for-sale yield after ten years	3.88%
Held-to-maturity amount within one year	1
Held-to-maturity yield within one year	7.79%
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	2
Held-to-maturity yield after five but within ten years	8.03%
Held-to-maturity amount after ten years	802
Held-to-maturity yield after ten years	6.14%
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	13
Available-for-sale yield after one but within five years	4.23%
Available-for-sale amount after five but within ten years	85
Available-for-sale yield after five but within ten years	2.40%
Available-for-sale amount after ten years	15,392
Available-for-sale yield after ten years	2.83%
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	7.73%
Held-to-maturity amount after ten years	251
Held-to-maturity yield after ten years	6.50%
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	101
Available-for-sale yield after one but within five years	3.98%
Available-for-sale amount after five but within ten years	305
Available-for-sale yield after five but within ten years	3.31%
Available-for-sale amount after ten years	335
Available-for-sale yield after ten years	3.46%
Held-to-maturity amount within one year	5
Held-to-maturity yield within one year	5.30%
Held-to-maturity amount after one but within five years	10
Held-to-maturity yield after one but within five years	4.56%
Held-to-maturity amount after five but within ten years	6
Held-to-maturity yield after five but within ten years	3.95%
Held-to-maturity amount after ten years	7
Held-to-maturity yield after ten years	5.05%
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	1.86%
Available-for-sale amount after five but within ten years	10
Available-for-sale yield after five but within ten years	0.37%
Available-for-sale amount after ten years	441
Available-for-sale yield after ten years	2.49%
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	18
Held-to-maturity yield after ten years	6.20%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	636
Available-for-sale yield within one year	3.01%
Available-for-sale amount after one but within five years	4,013
Available-for-sale yield after one but within five years	1.94%
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 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	63
Held-to-maturity yield after five but within ten years	0.33%
Held-to-maturity amount after ten years	$ 117
Held-to-maturity yield after ten years	0.33%