Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,978
Available-for-sale yield within one year (percent)	1.88%
Available-for-sale amount after one but within five years	$ 6,402
Available-for-sale yield after one but within five years (percent)	1.05%
Available-for-sale amount after five but within ten years	$ 5,919
Available-for-sale yield after five but within ten years (percent)	1.77%
Available-for-sale amount after ten years	$ 22,053
Available-for-sale yield after ten years (percent)	1.75%
Total fair value within one year	$ 1,981
Total fair value after one but within five years	6,513
Total fair value after five but within ten years	6,052
Total fair value after ten years	21,961
Held-to-maturity amount within one year	$ 6
Held-to-maturity yield within one year (percent)	3.78%
Held-to-maturity amount after one but within five years	$ 188
Held-to-maturity yield after one but within five years (percent)	2.66%
Held-to-maturity after five but within ten years	$ 337
Held-to-maturity yield after five but within ten years (percent)	2.34%
Held-to-maturity after ten years	$ 5,336
Held-to-maturity yield after ten years (percent)	2.67%
Total fair value within one year	$ 6
Total fair value after one but within five years	192
Total fair value after five but within ten years	350
Total fair value after ten years	5,549
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 772
Available-for-sale yield within one year (percent)	4.07%
Available-for-sale amount after one but within five years	$ 3,910
Available-for-sale yield after one but within five years (percent)	0.95%
Available-for-sale amount after five but within ten years	$ 3,666
Available-for-sale yield after five but within ten years (percent)	1.36%
Available-for-sale amount after ten years	$ 4,162
Available-for-sale yield after ten years (percent)	1.93%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 31
Available-for-sale yield within one year (percent)	2.75%
Available-for-sale amount after one but within five years	$ 1,316
Available-for-sale yield after one but within five years (percent)	1.78%
Available-for-sale amount after five but within ten years	$ 2,194
Available-for-sale yield after five but within ten years (percent)	2.35%
Available-for-sale amount after ten years	$ 7,474
Available-for-sale yield after ten years (percent)	1.64%
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	$ 184
Held-to-maturity yield after one but within five years (percent)	2.65%
Held-to-maturity after five but within ten years	$ 327
Held-to-maturity yield after five but within ten years (percent)	2.26%
Held-to-maturity after ten years	$ 774
Held-to-maturity yield after ten years (percent)	3.30%
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 71
Available-for-sale yield within one year (percent)	0.24%
Available-for-sale amount after one but within five years	$ 23
Available-for-sale yield after one but within five years (percent)	1.25%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years (percent)	6.13%
Available-for-sale amount after ten years	$ 10,346
Available-for-sale yield after ten years (percent)	1.75%
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year (percent)	3.83%
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	$ 8
Held-to-maturity yield after five but within ten years (percent)	5.14%
Held-to-maturity after ten years	$ 4,561
Held-to-maturity yield after ten years (percent)	2.56%
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year (percent)	3.43%
Held-to-maturity amount after one but within five years	$ 4
Held-to-maturity yield after one but within five years (percent)	3.00%
Held-to-maturity after five but within ten years	$ 2
Held-to-maturity yield after five but within ten years (percent)	4.25%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years (percent)	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 (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	$ 58
Available-for-sale yield after five but within ten years (percent)	5.13%
Available-for-sale amount after ten years	$ 71
Available-for-sale yield after ten years (percent)	3.52%
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)	7.53%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,104
Available-for-sale yield within one year (percent)	0.43%
Available-for-sale amount after one but within five years	$ 1,153
Available-for-sale yield after one but within five years (percent)	0.57%
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%