Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,795
Available-for-sale yield within one year	1.61%
Available-for-sale amount after one but within five years	$ 9,935
Available-for-sale yield after one but within five years	1.91%
Available-for-sale amount after five but within ten years	$ 9,981
Available-for-sale yield after five but within ten years	2.11%
Available-for-sale amount after ten years	$ 17,664
Available-for-sale yield after ten years	2.68%
Total fair value within one year	$ 3,800
Total fair value after one but within five years	9,970
Total fair value after five but within ten years	9,982
Total fair value after ten years	17,664
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	$ 409
Held-to-maturity yield after one but within five years	2.78%
Held-to-maturity after five but within ten years	$ 673
Held-to-maturity yield after five but within ten years	2.35%
Held-to-maturity after ten years	$ 12,268
Held-to-maturity yield after ten years	2.76%
Total fair value within one year	$ 0
Total fair value after one but within five years	415
Total fair value after five but within ten years	677
Total fair value after ten years	12,410
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 200
Available-for-sale yield within one year	2.22%
Available-for-sale amount after one but within five years	$ 7,920
Available-for-sale yield after one but within five years	1.79%
Available-for-sale amount after five but within ten years	$ 7,612
Available-for-sale yield after five but within ten years	1.91%
Available-for-sale amount after ten years	$ 3,694
Available-for-sale yield after ten years	2.49%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,746
Available-for-sale yield within one year	2.90%
Available-for-sale amount after one but within five years	$ 331
Available-for-sale yield after one but within five years	2.96%
Available-for-sale amount after five but within ten years	$ 2,309
Available-for-sale yield after five but within ten years	2.72%
Available-for-sale amount after ten years	$ 3,091
Available-for-sale yield after ten years	2.82%
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	$ 391
Held-to-maturity yield after one but within five years	2.74%
Held-to-maturity after five but within ten years	$ 648
Held-to-maturity yield after five but within ten years	2.30%
Held-to-maturity after ten years	$ 2,264
Held-to-maturity yield after ten years	3.17%
US Government agency issued or guaranteed [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	$ 80
Available-for-sale yield after one but within five years	2.19%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years	4.01%
Available-for-sale amount after ten years	$ 10,792
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	$ 13
Held-to-maturity yield after one but within five years	3.85%
Held-to-maturity after five but within ten years	$ 20
Held-to-maturity yield after five but within ten years	3.66%
Held-to-maturity after ten years	$ 10,002
Held-to-maturity yield after ten years	2.66%
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [Line Items]
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	$ 5
Held-to-maturity yield after one but within five years	3.49%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.22%
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	$ 59
Available-for-sale yield after five but within ten years	5.12%
Available-for-sale amount after ten years	$ 87
Available-for-sale yield after ten years	3.75%
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	7.61%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,849
Available-for-sale yield within one year	0.33%
Available-for-sale amount after one but within five years	$ 1,604
Available-for-sale yield after one but within five years	2.25%
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%