Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,090
Available-for-sale yield within one year	1.07%
Available-for-sale amount after one but within five years	$ 8,085
Available-for-sale yield after one but within five years	2.35%
Available-for-sale amount after five but within ten years	$ 9,269
Available-for-sale yield after five but within ten years	2.35%
Available-for-sale amount after ten years	$ 12,710
Available-for-sale yield after ten years	2.97%
Total fair value within one year	$ 2,095
Total fair value after one but within five years	8,006
Total fair value after five but within ten years	8,869
Total fair value after ten years	12,359
Held-to-maturity amount within one year	$ 51
Held-to-maturity yield within one year	2.26%
Held-to-maturity amount after one but within five years	$ 415
Held-to-maturity yield after one but within five years	2.77%
Held-to-maturity after five but within ten years	$ 187
Held-to-maturity yield after five but within ten years	2.85%
Held-to-maturity after ten years	$ 14,236
Held-to-maturity yield after ten years	2.69%
Total fair value within one year	$ 51
Total fair value after one but within five years	407
Total fair value after five but within ten years	181
Total fair value after ten years	13,806
U.S. Treasury [Member]
Debt Securities, Available-for-sale [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	$ 4,783
Available-for-sale yield after one but within five years	2.17%
Available-for-sale amount after five but within ten years	$ 6,968
Available-for-sale yield after five but within ten years	2.18%
Available-for-sale amount after ten years	$ 2,514
Available-for-sale yield after ten years	3.24%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 547
Available-for-sale yield within one year	3.88%
Available-for-sale amount after one but within five years	$ 2,273
Available-for-sale yield after one but within five years	2.81%
Available-for-sale amount after five but within ten years	$ 2,235
Available-for-sale yield after five but within ten years	2.79%
Available-for-sale amount after ten years	$ 4,242
Available-for-sale yield after ten years	3.15%
Held-to-maturity amount within one year	$ 50
Held-to-maturity yield within one year	2.21%
Held-to-maturity amount after one but within five years	$ 391
Held-to-maturity yield after one but within five years	2.71%
Held-to-maturity after five but within ten years	$ 155
Held-to-maturity yield after five but within ten years	2.66%
Held-to-maturity after ten years	$ 3,221
Held-to-maturity yield after ten years	2.95%
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 50
Available-for-sale yield within one year	1.85%
Available-for-sale amount after one but within five years	$ 71
Available-for-sale yield after one but within five years	2.22%
Available-for-sale amount after five but within ten years	$ 11
Available-for-sale yield after five but within ten years	3.64%
Available-for-sale amount after ten years	$ 5,864
Available-for-sale yield after ten years	2.71%
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	$ 19
Held-to-maturity yield after one but within five years	3.79%
Held-to-maturity after five but within ten years	$ 27
Held-to-maturity yield after five but within ten years	3.65%
Held-to-maturity after ten years	$ 11,013
Held-to-maturity yield after ten years	2.61%
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year	4.63%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.27%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.36%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years	0.00%
Asset-backed securities [Member]
Debt Securities, Available-for-sale [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	$ 55
Available-for-sale yield after five but within ten years	5.52%
Available-for-sale amount after ten years	$ 90
Available-for-sale yield after ten years	3.95%
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	$ 2
Held-to-maturity yield after ten years	8.84%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,493
Available-for-sale yield within one year	0.01%
Available-for-sale amount after one but within five years	$ 958
Available-for-sale yield after one but within five years	2.19%
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%