Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2022 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,799
Available-for-sale yield within one year (percent)	0.64%
Available-for-sale amount after one but within five years	$ 6,374
Available-for-sale yield after one but within five years (percent)	1.33%
Available-for-sale amount after five but within ten years	$ 4,057
Available-for-sale yield after five but within ten years (percent)	1.99%
Available-for-sale amount after ten years	$ 22,897
Available-for-sale yield after ten years (percent)	1.94%
Total fair value within one year	$ 1,801
Total fair value after one but within five years	6,278
Total fair value after five but within ten years	4,003
Total fair value after ten years	21,616
Held-to-maturity amount within one year	$ 7
Held-to-maturity yield within one year (percent)	2.93%
Held-to-maturity amount after one but within five years	$ 140
Held-to-maturity yield after one but within five years (percent)	2.56%
Held-to-maturity after five but within ten years	$ 371
Held-to-maturity yield after five but within ten years (percent)	2.67%
Held-to-maturity after ten years	$ 4,287
Held-to-maturity yield after ten years (percent)	2.66%
Total fair value within one year	$ 7
Total fair value after one but within five years	139
Total fair value after five but within ten years	370
Total fair value after ten years	4,242
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 247
Available-for-sale yield within one year (percent)	1.99%
Available-for-sale amount after one but within five years	$ 3,479
Available-for-sale yield after one but within five years (percent)	1.40%
Available-for-sale amount after five but within ten years	$ 1,967
Available-for-sale yield after five but within ten years (percent)	1.49%
Available-for-sale amount after ten years	$ 3,578
Available-for-sale yield after ten years (percent)	2.12%
U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 150
Available-for-sale yield within one year (percent)	1.41%
Available-for-sale amount after one but within five years	$ 1,730
Available-for-sale yield after one but within five years (percent)	1.67%
Available-for-sale amount after five but within ten years	$ 1,981
Available-for-sale yield after five but within ten years (percent)	2.38%
Available-for-sale amount after ten years	$ 7,108
Available-for-sale yield after ten years (percent)	1.66%
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year (percent)	2.55%
Held-to-maturity amount after one but within five years	$ 138
Held-to-maturity yield after one but within five years (percent)	2.54%
Held-to-maturity after five but within ten years	$ 360
Held-to-maturity yield after five but within ten years (percent)	2.58%
Held-to-maturity after ten years	$ 690
Held-to-maturity yield after ten years (percent)	3.28%
U.S. Government agency issued or guaranteed
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 70
Available-for-sale yield within one year (percent)	0.24%
Available-for-sale amount after one but within five years	$ 22
Available-for-sale yield after one but within five years (percent)	1.12%
Available-for-sale amount after five but within ten years	$ 3
Available-for-sale yield after five but within ten years (percent)	3.89%
Available-for-sale amount after ten years	$ 12,193
Available-for-sale yield after ten years (percent)	2.05%
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	$ 9
Held-to-maturity yield after five but within ten years (percent)	6.19%
Held-to-maturity after ten years	$ 3,596
Held-to-maturity yield after ten years (percent)	2.54%
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 4
Held-to-maturity yield within one year (percent)	3.22%
Held-to-maturity amount after one but within five years	$ 2
Held-to-maturity yield after one but within five years (percent)	3.54%
Held-to-maturity after five but within ten years	$ 2
Held-to-maturity yield after five but within ten years (percent)	4.34%
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	$ 106
Available-for-sale yield after five but within ten years (percent)	4.16%
Available-for-sale amount after ten years	$ 18
Available-for-sale yield after ten years (percent)	0.54%
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)	7.56%
Foreign debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,332
Available-for-sale yield within one year (percent)	0.32%
Available-for-sale amount after one but within five years	$ 1,143
Available-for-sale yield after one but within five years (percent)	0.60%
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%