Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2022 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,618
Available-for-sale yield within one year (percent)	0.66%
Available-for-sale amount after one but within five years	$ 6,022
Available-for-sale yield after one but within five years (percent)	1.56%
Available-for-sale amount after five but within ten years	$ 2,844
Available-for-sale yield after five but within ten years (percent)	2.00%
Available-for-sale amount after ten years	$ 20,596
Available-for-sale yield after ten years (percent)	2.25%
Total fair value within one year	$ 1,616
Total fair value after one but within five years	5,855
Total fair value after five but within ten years	2,696
Total fair value after ten years	17,915
Held-to-maturity amount within one year	$ 16
Held-to-maturity yield within one year (percent)	2.95%
Held-to-maturity amount after one but within five years	$ 96
Held-to-maturity yield after one but within five years (percent)	2.39%
Held-to-maturity after five but within ten years	$ 866
Held-to-maturity yield after five but within ten years (percent)	3.18%
Held-to-maturity after ten years	$ 5,678
Held-to-maturity yield after ten years (percent)	3.23%
Total fair value within one year	$ 16
Total fair value after one but within five years	91
Total fair value after five but within ten years	799
Total fair value after ten years	5,275
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 249
Available-for-sale yield within one year (percent)	2.00%
Available-for-sale amount after one but within five years	$ 3,859
Available-for-sale yield after one but within five years (percent)	1.51%
Available-for-sale amount after five but within ten years	$ 1,286
Available-for-sale yield after five but within ten years (percent)	1.54%
Available-for-sale amount after ten years	$ 2,933
Available-for-sale yield after ten years (percent)	2.71%
U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 192
Available-for-sale yield within one year (percent)	1.17%
Available-for-sale amount after one but within five years	$ 1,725
Available-for-sale yield after one but within five years (percent)	1.74%
Available-for-sale amount after five but within ten years	$ 1,451
Available-for-sale yield after five but within ten years (percent)	2.25%
Available-for-sale amount after ten years	$ 6,659
Available-for-sale yield after ten years (percent)	1.75%
Held-to-maturity amount within one year	$ 14
Held-to-maturity yield within one year (percent)	2.84%
Held-to-maturity amount after one but within five years	$ 92
Held-to-maturity yield after one but within five years (percent)	2.34%
Held-to-maturity after five but within ten years	$ 855
Held-to-maturity yield after five but within ten years (percent)	3.14%
Held-to-maturity after ten years	$ 588
Held-to-maturity yield after ten years (percent)	3.38%
U.S. Government agency issued or guaranteed
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 0
Available-for-sale yield within one year (percent)	0.00%
Available-for-sale amount after one but within five years	$ 21
Available-for-sale yield after one but within five years (percent)	0.96%
Available-for-sale amount after five but within ten years	$ 2
Available-for-sale yield after five but within ten years (percent)	6.35%
Available-for-sale amount after ten years	$ 10,988
Available-for-sale yield after ten years (percent)	2.42%
Held-to-maturity amount within one year	$ 0
Held-to-maturity yield within one year (percent)	0.00%
Held-to-maturity amount after one but within five years	$ 2
Held-to-maturity yield after one but within five years (percent)	2.44%
Held-to-maturity after five but within ten years	$ 9
Held-to-maturity yield after five but within ten years (percent)	6.82%
Held-to-maturity after ten years	$ 5,089
Held-to-maturity yield after ten years (percent)	3.21%
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year (percent)	3.68%
Held-to-maturity amount after one but within five years	$ 2
Held-to-maturity yield after one but within five years (percent)	4.45%
Held-to-maturity after five but within ten years	$ 2
Held-to-maturity yield after five but within ten years (percent)	2.42%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years (percent)	0.00%
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 0
Available-for-sale yield within one year (percent)	0.00%
Available-for-sale amount after one but within five years	$ 0
Available-for-sale yield after one but within five years (percent)	0.00%
Available-for-sale amount after five but within ten years	$ 105
Available-for-sale yield after five but within ten years (percent)	4.17%
Available-for-sale amount after ten years	$ 16
Available-for-sale yield after ten years (percent)	2.94%
Held-to-maturity amount within one year	$ 0
Held-to-maturity yield within one year (percent)	0.00%
Held-to-maturity amount after one but within five years	$ 0
Held-to-maturity yield after one but within five years (percent)	0.00%
Held-to-maturity after five but within ten years	$ 0
Held-to-maturity yield after five but within ten years (percent)	0.00%
Held-to-maturity after ten years	$ 1
Held-to-maturity yield after ten years (percent)	7.68%
Foreign debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,177
Available-for-sale yield within one year (percent)	0.29%
Available-for-sale amount after one but within five years	$ 417
Available-for-sale yield after one but within five years (percent)	1.30%
Available-for-sale amount after five but within ten years	$ 0
Available-for-sale yield after five but within ten years (percent)	0.00%
Available-for-sale amount after ten years	$ 0
Available-for-sale yield after ten years (percent)	0.00%