Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 7,118
Available-for-sale yield within one year	0.63%
Available-for-sale amount after one but within five years	27,293
Available-for-sale yield after one but within five years	0.76%
Available-for-sale amount after five but within ten years	7,096
Available-for-sale yield after five but within ten years	2.58%
Available-for-sale amount after ten years	24,367
Available-for-sale yield after ten years	3.08%
Total fair value within one year	7,125
Total fair value after one but within five years	27,344
Total fair value after five but within ten years	7,689
Total fair value after ten years	25,385
Held-to-maturity amount within one year	4
Held-to-maturity yield within one year	5.82%
Held-to-maturity amount after one but within five years	22
Held-to-maturity yield after one but within five years	5.96%
Held-to-maturity amount after five but within ten years	13
Held-to-maturity yield after five but within ten years	5.34%
Held-to-maturity amount after ten years	1,581
Held-to-maturity yield after ten years	6.23%
Total fair value within one year	4
Total fair value after one but within five years	23
Total fair value after five but within ten years	14
Total fair value after ten years	1,790
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	6,505
Available-for-sale yield within one year	0.30%
Available-for-sale amount after one but within five years	22,173
Available-for-sale yield after one but within five years	0.49%
Available-for-sale amount after five but within ten years	3,526
Available-for-sale yield after five but within ten years	2.07%
Available-for-sale amount after ten years	2,596
Available-for-sale yield after ten years	3.04%
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	296
Available-for-sale yield after one but within five years	2.78%
Available-for-sale amount after five but within ten years	3,053
Available-for-sale yield after five but within ten years	3.06%
Available-for-sale amount after ten years	856
Available-for-sale yield after ten years	3.51%
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	6
Held-to-maturity yield after one but within five years	7.75%
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	1,113
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	6
Available-for-sale yield after one but within five years	4.66%
Available-for-sale amount after five but within ten years	92
Available-for-sale yield after five but within ten years	1.87%
Available-for-sale amount after ten years	19,864
Available-for-sale yield after ten years	3.06%
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	7.58%
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	339
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 within one year	0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	45
Available-for-sale yield after one but within five years	4.15%
Available-for-sale amount after five but within ten years	414
Available-for-sale yield after five but within ten years	3.63%
Available-for-sale amount after ten years	418
Available-for-sale yield after ten years	3.67%
Held-to-maturity amount within one year	3
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	5.08%
Held-to-maturity amount after five but within ten years	9
Held-to-maturity yield after five but within ten years	4.31%
Held-to-maturity amount after ten years	11
Held-to-maturity yield after ten years	5.00%
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	1.95%
Available-for-sale amount after five but within ten years	11
Available-for-sale yield after five but within ten years	0.38%
Available-for-sale amount after ten years	609
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	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	118
Held-to-maturity yield after ten years	6.27%
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	3.90%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	613
Available-for-sale yield within one year	4.15%
Available-for-sale amount after one but within five years	4,772
Available-for-sale yield after one but within five years	1.84%
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%