Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 2,717
Available-for-sale yield within one year	1.16%
Available-for-sale amount after one but within five years	24,683
Available-for-sale yield after one but within five years	0.79%
Available-for-sale amount after five but within ten years	6,272
Available-for-sale yield after five but within ten years	2.87%
Available-for-sale amount after ten years	24,201
Available-for-sale yield after ten years	2.90%
Total fair value within one year	2,721
Total fair value after one but within five years	24,769
Total fair value after five but within ten years	6,794
Total fair value after ten years	25,002
Held-to-maturity amount within one year	6
Held-to-maturity yield within one year	5.57%
Held-to-maturity amount after one but within five years	20
Held-to-maturity yield after one but within five years	5.87%
Held-to-maturity amount after five but within ten years	13
Held-to-maturity yield after five but within ten years	5.41%
Held-to-maturity amount after ten years	1,395
Held-to-maturity yield after ten years	6.23%
Total fair value within one year	6
Total fair value after one but within five years	20
Total fair value after five but within ten years	14
Total fair value after ten years	1,594
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	0.41%
Available-for-sale amount after one but within five years	19,694
Available-for-sale yield after one but within five years	0.48%
Available-for-sale amount after five but within ten years	2,880
Available-for-sale yield after five but within ten years	2.60%
Available-for-sale amount after ten years	2,139
Available-for-sale yield after ten years	3.57%
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	0.39%
Available-for-sale amount after one but within five years	333
Available-for-sale yield after one but within five years	3.26%
Available-for-sale amount after five but within ten years	3,000
Available-for-sale yield after five but within ten years	3.05%
Available-for-sale amount after ten years	822
Available-for-sale yield after ten years	3.64%
Held-to-maturity amount within one year	1
Held-to-maturity yield within one year	7.98%
Held-to-maturity amount after one but within five years	5
Held-to-maturity yield after one but within five years	7.68%
Held-to-maturity amount after five but within ten years	1
Held-to-maturity yield after five but within ten years	7.89%
Held-to-maturity amount after ten years	1,028
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	4
Available-for-sale yield after one but within five years	4.66%
Available-for-sale amount after five but within ten years	88
Available-for-sale yield after five but within ten years	1.88%
Available-for-sale amount after ten years	20,245
Available-for-sale yield after ten years	2.79%
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	7.59%
Held-to-maturity amount after five but within ten years	3
Held-to-maturity yield after five but within ten years	7.69%
Held-to-maturity amount after ten years	319
Held-to-maturity yield after ten years	6.50%
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	49
Available-for-sale yield after one but within five years	4.20%
Available-for-sale amount after five but within ten years	293
Available-for-sale yield after five but within ten years	4.01%
Available-for-sale amount after ten years	401
Available-for-sale yield after ten years	3.68%
Held-to-maturity amount within one year	5
Held-to-maturity yield within one year	5.05%
Held-to-maturity amount after one but within five years	14
Held-to-maturity yield after one but within five years	5.13%
Held-to-maturity amount after five but within ten years	9
Held-to-maturity yield after five but within ten years	4.42%
Held-to-maturity amount after ten years	9
Held-to-maturity yield after ten years	5.02%
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	1.83%
Available-for-sale amount after five but within ten years	11
Available-for-sale yield after five but within ten years	0.41%
Available-for-sale amount after ten years	570
Available-for-sale yield after ten years	2.77%
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	39
Held-to-maturity yield after ten years	6.41%
Corporate and other domestic [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	24
Available-for-sale yield after ten years	4.00%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	665
Available-for-sale yield within one year	3.48%
Available-for-sale amount after one but within five years	4,602
Available-for-sale yield after one but within five years	1.87%
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%