Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 1,699
Available-for-sale yield within one year	1.04%
Available-for-sale amount after one but within five years	$ 10,544
Available-for-sale yield after one but within five years	1.70%
Available-for-sale amount after five but within ten years	$ 5,024
Available-for-sale yield after five but within ten years	2.34%
Available-for-sale amount after ten years	$ 18,425
Available-for-sale yield after ten years	2.73%
Total fair value within one year	$ 1,704
Total fair value after one but within five years	10,663
Total fair value after five but within ten years	5,117
Total fair value after ten years	18,287
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.04%
Held-to-maturity amount after one but within five years	$ 100
Held-to-maturity yield after one but within five years	1.67%
Held-to-maturity after five but within ten years	$ 666
Held-to-maturity yield after five but within ten years	2.66%
Held-to-maturity after ten years	$ 13,711
Held-to-maturity yield after ten years	2.55%
Total fair value within one year	$ 2
Total fair value after one but within five years	102
Total fair value after five but within ten years	685
Total fair value after ten years	13,978
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 1,321
Available-for-sale yield within one year	0.83%
Available-for-sale amount after one but within five years	$ 4,540
Available-for-sale yield after one but within five years	1.14%
Available-for-sale amount after five but within ten years	$ 3,539
Available-for-sale yield after five but within ten years	2.27%
Available-for-sale amount after ten years	$ 3,256
Available-for-sale yield after ten years	3.01%
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	$ 2,753
Available-for-sale yield after one but within five years	2.98%
Available-for-sale amount after five but within ten years	$ 1,201
Available-for-sale yield after five but within ten years	2.42%
Available-for-sale amount after ten years	$ 1,027
Available-for-sale yield after ten years	2.96%
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	1.40%
Held-to-maturity after five but within ten years	$ 618
Held-to-maturity yield after five but within ten years	2.62%
Held-to-maturity after ten years	$ 4,161
Held-to-maturity yield after ten years	2.95%
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	$ 6
Available-for-sale yield after one but within five years	4.20%
Available-for-sale amount after five but within ten years	$ 35
Available-for-sale yield after five but within ten years	3.88%
Available-for-sale amount after ten years	$ 13,480
Available-for-sale yield after ten years	2.62%
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	7.75%
Held-to-maturity after five but within ten years	$ 43
Held-to-maturity yield after five but within ten years	3.02%
Held-to-maturity after ten years	$ 9,537
Held-to-maturity yield after ten years	2.38%
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	$ 72
Available-for-sale yield after one but within five years	4.21%
Available-for-sale amount after five but within ten years	$ 249
Available-for-sale yield after five but within ten years	2.62%
Available-for-sale amount after ten years	$ 442
Available-for-sale yield after ten years	3.36%
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.04%
Held-to-maturity amount after one but within five years	$ 7
Held-to-maturity yield after one but within five years	4.09%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.41%
Held-to-maturity after ten years	$ 5
Held-to-maturity yield after ten years	5.32%
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	$ 220
Available-for-sale yield after ten years	3.26%
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	$ 8
Held-to-maturity yield after ten years	6.50%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 378
Available-for-sale yield within one year	1.78%
Available-for-sale amount after one but within five years	$ 3,173
Available-for-sale yield after one but within five years	1.33%
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%