Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 795
Available-for-sale yield within one year	1.95%
Available-for-sale amount after one but within five years	$ 9,278
Available-for-sale yield after one but within five years	1.77%
Available-for-sale amount after five but within ten years	$ 11,785
Available-for-sale yield after five but within ten years	2.15%
Available-for-sale amount after ten years	$ 18,402
Available-for-sale yield after ten years	2.37%
Total fair value within one year	$ 799
Total fair value after one but within five years	9,488
Total fair value after five but within ten years	12,025
Total fair value after ten years	18,330
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year	3.71%
Held-to-maturity amount after one but within five years	$ 225
Held-to-maturity yield after one but within five years	2.06%
Held-to-maturity after five but within ten years	$ 525
Held-to-maturity yield after five but within ten years	2.75%
Held-to-maturity after ten years	$ 12,678
Held-to-maturity yield after ten years	2.48%
Total fair value within one year	$ 3
Total fair value after one but within five years	229
Total fair value after five but within ten years	550
Total fair value after ten years	12,980
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity 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	$ 5,733
Available-for-sale yield after one but within five years	1.20%
Available-for-sale amount after five but within ten years	$ 10,427
Available-for-sale yield after five but within ten years	2.12%
Available-for-sale amount after ten years	$ 5,322
Available-for-sale yield after ten years	2.56%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 198
Available-for-sale yield within one year	3.26%
Available-for-sale amount after one but within five years	$ 3,281
Available-for-sale yield after one but within five years	2.83%
Available-for-sale amount after five but within ten years	$ 1,331
Available-for-sale yield after five but within ten years	2.35%
Available-for-sale amount after ten years	$ 5,087
Available-for-sale yield after ten years	2.13%
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	$ 213
Held-to-maturity yield after one but within five years	2.05%
Held-to-maturity after five but within ten years	$ 484
Held-to-maturity yield after five but within ten years	2.70%
Held-to-maturity after ten years	$ 3,535
Held-to-maturity yield after ten years	2.88%
US Government agency issued or guaranteed [Member]
Schedule of Available-for-sale and Held-to-Maturity 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	$ 3
Available-for-sale yield after one but within five years	4.21%
Available-for-sale amount after five but within ten years	$ 27
Available-for-sale yield after five but within ten years	3.81%
Available-for-sale amount after ten years	$ 7,798
Available-for-sale yield after ten years	2.37%
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	$ 8
Held-to-maturity yield after one but within five years	1.56%
Held-to-maturity after five but within ten years	$ 36
Held-to-maturity yield after five but within ten years	3.24%
Held-to-maturity after ten years	$ 9,133
Held-to-maturity yield after ten years	2.32%
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year	3.71%
Held-to-maturity amount after one but within five years	$ 4
Held-to-maturity yield after one but within five years	3.51%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.73%
Held-to-maturity after ten years	$ 4
Held-to-maturity yield after ten years	5.61%
Asset backed securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 400
Available-for-sale yield within one year	1.67%
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	$ 195
Available-for-sale yield after ten years	3.24%
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	$ 6
Held-to-maturity yield after ten years	6.41%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 197
Available-for-sale yield within one year	1.21%
Available-for-sale amount after one but within five years	$ 261
Available-for-sale yield after one but within five years	0.87%
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%