Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,439
Available-for-sale yield within one year (percent)	0.43%
Available-for-sale amount after one but within five years	$ 6,288
Available-for-sale yield after one but within five years (percent)	1.30%
Available-for-sale amount after five but within ten years	$ 4,262
Available-for-sale yield after five but within ten years (percent)	1.93%
Available-for-sale amount after ten years	$ 23,367
Available-for-sale yield after ten years (percent)	1.78%
Total fair value within one year	$ 1,442
Total fair value after one but within five years	6,340
Total fair value after five but within ten years	4,346
Total fair value after ten years	23,170
Held-to-maturity amount within one year	$ 7
Held-to-maturity yield within one year (percent)	3.33%
Held-to-maturity amount after one but within five years	$ 182
Held-to-maturity yield after one but within five years (percent)	2.65%
Held-to-maturity after five but within ten years	$ 309
Held-to-maturity yield after five but within ten years (percent)	2.36%
Held-to-maturity after ten years	$ 4,706
Held-to-maturity yield after ten years (percent)	2.66%
Total fair value within one year	$ 8
Total fair value after one but within five years	184
Total fair value after five but within ten years	318
Total fair value after ten years	4,849
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 246
Available-for-sale yield within one year (percent)	1.99%
Available-for-sale amount after one but within five years	$ 3,364
Available-for-sale yield after one but within five years (percent)	1.38%
Available-for-sale amount after five but within ten years	$ 2,075
Available-for-sale yield after five but within ten years (percent)	1.39%
Available-for-sale amount after ten years	$ 3,805
Available-for-sale yield after ten years (percent)	1.87%
U.S. Government sponsored enterprises
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	$ 1,712
Available-for-sale yield after one but within five years (percent)	1.63%
Available-for-sale amount after five but within ten years	$ 2,079
Available-for-sale yield after five but within ten years (percent)	2.35%
Available-for-sale amount after ten years	$ 7,136
Available-for-sale yield after ten years (percent)	1.64%
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year (percent)	2.69%
Held-to-maturity amount after one but within five years	$ 179
Held-to-maturity yield after one but within five years (percent)	2.64%
Held-to-maturity after five but within ten years	$ 299
Held-to-maturity yield after five but within ten years (percent)	2.26%
Held-to-maturity after ten years	$ 695
Held-to-maturity yield after ten years (percent)	3.31%
U.S. Government agency issued or guaranteed
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)	1.17%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years (percent)	5.54%
Available-for-sale amount after ten years	$ 12,406
Available-for-sale yield after ten years (percent)	1.83%
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year (percent)	3.51%
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)	5.38%
Held-to-maturity after ten years	$ 4,010
Held-to-maturity yield after ten years (percent)	2.55%
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year (percent)	3.92%
Held-to-maturity amount after one but within five years	$ 3
Held-to-maturity yield after one but within five years (percent)	2.68%
Held-to-maturity after five but within ten years	$ 2
Held-to-maturity yield after five but within ten years (percent)	4.32%
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	$ 107
Available-for-sale yield after five but within ten years (percent)	4.14%
Available-for-sale amount after ten years	$ 20
Available-for-sale yield after ten years (percent)	0.23%
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)	7.52%
Foreign debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,122
Available-for-sale yield within one year (percent)	0.10%
Available-for-sale amount after one but within five years	$ 1,189
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%