Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 1,263
Available-for-sale yield within one year	1.05%
Available-for-sale amount after one but within five years	13,267
Available-for-sale yield after one but within five years	1.43%
Available-for-sale amount after five but within ten years	3,216
Available-for-sale yield after five but within ten years	3.21%
Available-for-sale amount after ten years	11,743
Available-for-sale yield after ten years	2.97%
Total fair value within one year	1,270
Total fair value after one but within five years	13,341
Total fair value after five but within ten years	3,428
Total fair value after ten years	11,934
Held-to-maturity amount within one year	3
Held-to-maturity yield within one year	5.02%
Held-to-maturity amount after one but within five years	104
Held-to-maturity yield after one but within five years	1.66%
Held-to-maturity after five but within ten years	461
Held-to-maturity yield after five but within ten years	2.84%
Held-to-maturity after ten years	12,901
Held-to-maturity yield after ten years	2.66%
Total fair value within one year	3
Total fair value after one but within five years	103
Total fair value after five but within ten years	472
Total fair value after ten years	13,117
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	1,155
Available-for-sale yield within one year	0.87%
Available-for-sale amount after one but within five years	7,387
Available-for-sale yield after one but within five years	0.84%
Available-for-sale amount after five but within ten years	1,457
Available-for-sale yield after five but within ten years	3.20%
Available-for-sale amount after ten years	1,794
Available-for-sale yield after ten years	3.41%
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	1,944
Available-for-sale yield after one but within five years	3.16%
Available-for-sale amount after five but within ten years	1,417
Available-for-sale yield after five but within ten years	3.24%
Available-for-sale amount after ten years	1,189
Available-for-sale yield after ten years	3.19%
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	406
Held-to-maturity yield after five but within ten years	2.85%
Held-to-maturity after ten years	4,370
Held-to-maturity yield after ten years	3.10%
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	10
Available-for-sale yield after one but within five years	4.22%
Available-for-sale amount after five but within ten years	44
Available-for-sale yield after five but within ten years	3.89%
Available-for-sale amount after ten years	8,260
Available-for-sale yield after ten years	2.81%
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	2
Held-to-maturity yield after one but within five years	7.64%
Held-to-maturity after five but within ten years	49
Held-to-maturity yield after five but within ten years	2.64%
Held-to-maturity after ten years	8,516
Held-to-maturity yield after ten years	2.42%
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	113
Available-for-sale yield after one but within five years	4.13%
Available-for-sale amount after five but within ten years	298
Available-for-sale yield after five but within ten years	2.99%
Available-for-sale amount after ten years	250
Available-for-sale yield after ten years	3.69%
Held-to-maturity amount within one year	3
Held-to-maturity yield within one year	5.00%
Held-to-maturity amount after one but within five years	10
Held-to-maturity yield after one but within five years	3.95%
Held-to-maturity after five but within ten years	6
Held-to-maturity yield after five but within ten years	3.83%
Held-to-maturity after ten years	4
Held-to-maturity yield after ten years	5.35%
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	250
Available-for-sale yield after ten years	3.33%
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	11
Held-to-maturity yield after ten years	6.39%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	108
Available-for-sale yield within one year	2.92%
Available-for-sale amount after one but within five years	3,813
Available-for-sale yield after one but within five years	1.60%
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%