Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 1,102
Available-for-sale yield within one year	1.23%
Available-for-sale amount after one but within five years	$ 6,607
Available-for-sale yield after one but within five years	2.48%
Available-for-sale amount after five but within ten years	$ 10,415
Available-for-sale yield after five but within ten years	2.06%
Available-for-sale amount after ten years	$ 13,640
Available-for-sale yield after ten years	2.69%
Total fair value within one year	$ 1,103
Total fair value after one but within five years	6,727
Total fair value after five but within ten years	10,233
Total fair value after ten years	13,476
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.17%
Held-to-maturity amount after one but within five years	$ 340
Held-to-maturity yield after one but within five years	2.48%
Held-to-maturity after five but within ten years	$ 374
Held-to-maturity yield after five but within ten years	2.77%
Held-to-maturity after ten years	$ 13,645
Held-to-maturity yield after ten years	2.46%
Total fair value within one year	$ 2
Total fair value after one but within five years	342
Total fair value after five but within ten years	379
Total fair value after ten years	13,678
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	$ 3,215
Available-for-sale yield after one but within five years	2.12%
Available-for-sale amount after five but within ten years	$ 8,893
Available-for-sale yield after five but within ten years	1.97%
Available-for-sale amount after ten years	$ 3,961
Available-for-sale yield after ten years	2.84%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 99
Available-for-sale yield within one year	3.26%
Available-for-sale amount after one but within five years	$ 2,921
Available-for-sale yield after one but within five years	3.00%
Available-for-sale amount after five but within ten years	$ 1,502
Available-for-sale yield after five but within ten years	2.52%
Available-for-sale amount after ten years	$ 2,866
Available-for-sale yield after ten years	2.70%
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	$ 310
Held-to-maturity yield after one but within five years	2.36%
Held-to-maturity after five but within ten years	$ 339
Held-to-maturity yield after five but within ten years	2.77%
Held-to-maturity after ten years	$ 2,888
Held-to-maturity yield after ten years	2.86%
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	$ 1
Available-for-sale yield within one year	3.49%
Available-for-sale amount after one but within five years	$ 183
Available-for-sale yield after one but within five years	2.56%
Available-for-sale amount after five but within ten years	$ 20
Available-for-sale yield after five but within ten years	3.91%
Available-for-sale amount after ten years	$ 6,648
Available-for-sale yield after ten years	2.57%
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	$ 26
Held-to-maturity yield after one but within five years	3.78%
Held-to-maturity after five but within ten years	$ 29
Held-to-maturity yield after five but within ten years	2.47%
Held-to-maturity after ten years	$ 10,751
Held-to-maturity yield after ten years	2.35%
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	$ 2
Held-to-maturity yield within one year	4.17%
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	$ 6
Held-to-maturity yield after five but within ten years	4.15%
Held-to-maturity after ten years	$ 2
Held-to-maturity yield after ten years	4.16%
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	2.56%
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	$ 165
Available-for-sale yield after ten years	3.85%
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	$ 4
Held-to-maturity yield after ten years	7.11%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 602
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 288
Available-for-sale yield after one but within five years	1.18%
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%