Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,308
Available-for-sale yield within one year	1.36%
Available-for-sale amount after one but within five years	$ 5,336
Available-for-sale yield after one but within five years	1.44%
Available-for-sale amount after five but within ten years	$ 8,215
Available-for-sale yield after five but within ten years	1.79%
Available-for-sale amount after ten years	$ 17,602
Available-for-sale yield after ten years	2.02%
Total fair value within one year	$ 3,326
Total fair value after one but within five years	5,454
Total fair value after five but within ten years	8,286
Total fair value after ten years	17,593
Held-to-maturity amount within one year	$ 75
Held-to-maturity yield within one year	2.39%
Held-to-maturity amount after one but within five years	$ 255
Held-to-maturity yield after one but within five years	2.78%
Held-to-maturity after five but within ten years	$ 410
Held-to-maturity yield after five but within ten years	2.31%
Held-to-maturity after ten years	$ 7,099
Held-to-maturity yield after ten years	2.66%
Total fair value within one year	$ 75
Total fair value after one but within five years	265
Total fair value after five but within ten years	425
Total fair value after ten years	7,401
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,437
Available-for-sale yield within one year	2.41%
Available-for-sale amount after one but within five years	$ 3,097
Available-for-sale yield after one but within five years	1.56%
Available-for-sale amount after five but within ten years	$ 5,384
Available-for-sale yield after five but within ten years	1.35%
Available-for-sale amount after ten years	$ 3,656
Available-for-sale yield after ten years	2.27%
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	2.71%
Available-for-sale amount after one but within five years	$ 966
Available-for-sale yield after one but within five years	1.94%
Available-for-sale amount after five but within ten years	$ 2,771
Available-for-sale yield after five but within ten years	2.56%
Available-for-sale amount after ten years	$ 6,154
Available-for-sale yield after ten years	1.74%
Held-to-maturity amount within one year	$ 74
Held-to-maturity yield within one year	2.39%
Held-to-maturity amount after one but within five years	$ 242
Held-to-maturity yield after one but within five years	2.71%
Held-to-maturity after five but within ten years	$ 397
Held-to-maturity yield after five but within ten years	2.24%
Held-to-maturity after ten years	$ 988
Held-to-maturity yield after ten years	3.27%
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	$ 96
Available-for-sale yield after one but within five years	1.79%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years	7.87%
Available-for-sale amount after ten years	$ 7,717
Available-for-sale yield after ten years	2.12%
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	$ 8
Held-to-maturity yield after one but within five years	4.02%
Held-to-maturity after five but within ten years	$ 11
Held-to-maturity yield after five but within ten years	4.76%
Held-to-maturity after ten years	$ 6,109
Held-to-maturity yield after ten years	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	$ 1
Held-to-maturity yield within one year	2.72%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.47%
Held-to-maturity after five but within ten years	$ 2
Held-to-maturity yield after five but within ten years	4.21%
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	5.06%
Available-for-sale amount after ten years	$ 75
Available-for-sale yield after ten years	3.36%
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	5.85%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,840
Available-for-sale yield within one year	0.51%
Available-for-sale amount after one but within five years	$ 1,177
Available-for-sale yield after one but within five years	0.68%
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%