Securities - Distribution of Maturities of Debt Securities with Approximate Yield of Portfolio (Detail) $ in Millions	Sep. 30, 2024 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,667
Available-for-sale yield within one year (percent)	0.58%
Available-for-sale amount after one but within five years	$ 6,101
Available-for-sale yield after one but within five years (percent)	2.95%
Available-for-sale amount after five but within ten years	$ 3,170
Available-for-sale yield after five but within ten years (percent)	3.55%
Available-for-sale amount after ten years	$ 15,808
Available-for-sale yield after ten years (percent)	2.54%
Total fair value within one year	$ 2,664
Total fair value after one but within five years	6,057
Total fair value after five but within ten years	3,153
Total fair value after ten years	14,068
Held-to-maturity amount within one year	$ 85
Held-to-maturity yield within one year (percent)	2.32%
Held-to-maturity amount after one but within five years	$ 2,270
Held-to-maturity yield after one but within five years (percent)	3.57%
Held-to-maturity after five but within ten years	$ 1,371
Held-to-maturity yield after five but within ten years (percent)	3.73%
Held-to-maturity after ten years	$ 13,512
Held-to-maturity yield after ten years (percent)	4.48%
Total fair value within one year	$ 85
Total fair value after one but within five years	2,266
Total fair value after five but within ten years	1,346
Total fair value after ten years	13,339
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 243
Available-for-sale yield within one year (percent)	0.98%
Available-for-sale amount after one but within five years	$ 3,815
Available-for-sale yield after one but within five years (percent)	2.17%
Available-for-sale amount after five but within ten years	$ 2,380
Available-for-sale yield after five but within ten years (percent)	3.97%
Available-for-sale amount after ten years	$ 2,880
Available-for-sale yield after ten years (percent)	2.65%
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	$ 2,048
Held-to-maturity yield after one but within five years (percent)	3.65%
Held-to-maturity after five but within ten years	$ 749
Held-to-maturity yield after five but within ten years (percent)	3.71%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years (percent)	0.00%
U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 579
Available-for-sale yield within one year (percent)	1.15%
Available-for-sale amount after one but within five years	$ 178
Available-for-sale yield after one but within five years (percent)	2.99%
Available-for-sale amount after five but within ten years	$ 650
Available-for-sale yield after five but within ten years (percent)	1.97%
Available-for-sale amount after ten years	$ 4,673
Available-for-sale yield after ten years (percent)	1.80%
Held-to-maturity amount within one year	$ 84
Held-to-maturity yield within one year (percent)	2.30%
Held-to-maturity amount after one but within five years	$ 219
Held-to-maturity yield after one but within five years (percent)	2.82%
Held-to-maturity after five but within ten years	$ 610
Held-to-maturity yield after five but within ten years (percent)	3.70%
Held-to-maturity after ten years	$ 364
Held-to-maturity yield after ten years (percent)	2.86%
U.S. Government agency issued or guaranteed
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 20
Available-for-sale yield within one year (percent)	0.81%
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	$ 2
Available-for-sale yield after five but within ten years (percent)	4.65%
Available-for-sale amount after ten years	$ 8,243
Available-for-sale yield after ten years (percent)	2.92%
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	$ 1
Held-to-maturity yield after one but within five years (percent)	3.94%
Held-to-maturity after five but within ten years	$ 12
Held-to-maturity yield after five but within ten years (percent)	5.91%
Held-to-maturity after ten years	$ 13,148
Held-to-maturity yield after ten years (percent)	4.52%
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year (percent)	4.04%
Held-to-maturity amount after one but within five years	$ 2
Held-to-maturity yield after one but within five years (percent)	3.77%
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	$ 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	$ 54
Available-for-sale yield after one but within five years (percent)	4.40%
Available-for-sale amount after five but within ten years	$ 47
Available-for-sale yield after five but within ten years (percent)	3.93%
Available-for-sale amount after ten years	$ 12
Available-for-sale yield after ten years (percent)	4.85%
Foreign debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,825
Available-for-sale yield within one year (percent)	0.34%
Available-for-sale amount after one but within five years	$ 2,054
Available-for-sale yield after one but within five years (percent)	4.36%
Available-for-sale amount after five but within ten years	$ 91
Available-for-sale yield after five but within ten years (percent)	3.81%
Available-for-sale amount after ten years	$ 0
Available-for-sale yield after ten years (percent)	0.00%