Securities - Distribution of Maturities of Debt Securities with Approximate Yield of Portfolio (Detail) $ in Millions	Mar. 31, 2024 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,440
Available-for-sale yield within one year (percent)	0.67%
Available-for-sale amount after one but within five years	$ 6,319
Available-for-sale yield after one but within five years (percent)	2.87%
Available-for-sale amount after five but within ten years	$ 2,159
Available-for-sale yield after five but within ten years (percent)	2.32%
Available-for-sale amount after ten years	$ 17,054
Available-for-sale yield after ten years (percent)	2.58%
Total fair value within one year	$ 2,419
Total fair value after one but within five years	6,236
Total fair value after five but within ten years	2,068
Total fair value after ten years	14,827
Held-to-maturity amount within one year	$ 86
Held-to-maturity yield within one year (percent)	2.34%
Held-to-maturity amount after one but within five years	$ 1,269
Held-to-maturity yield after one but within five years (percent)	3.74%
Held-to-maturity after five but within ten years	$ 1,537
Held-to-maturity yield after five but within ten years (percent)	3.60%
Held-to-maturity after ten years	$ 12,402
Held-to-maturity yield after ten years (percent)	4.45%
Total fair value within one year	$ 85
Total fair value after one but within five years	1,240
Total fair value after five but within ten years	1,458
Total fair value after ten years	11,934
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 237
Available-for-sale yield within one year (percent)	1.00%
Available-for-sale amount after one but within five years	$ 3,406
Available-for-sale yield after one but within five years (percent)	1.98%
Available-for-sale amount after five but within ten years	$ 833
Available-for-sale yield after five but within ten years (percent)	2.46%
Available-for-sale amount after ten years	$ 2,741
Available-for-sale yield after ten years (percent)	2.82%
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	$ 1,185
Held-to-maturity yield after one but within five years (percent)	3.74%
Held-to-maturity after five but within ten years	$ 742
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	$ 820
Available-for-sale yield within one year (percent)	1.03%
Available-for-sale amount after one but within five years	$ 979
Available-for-sale yield after one but within five years (percent)	2.73%
Available-for-sale amount after five but within ten years	$ 1,276
Available-for-sale yield after five but within ten years (percent)	2.16%
Available-for-sale amount after ten years	$ 5,707
Available-for-sale yield after ten years (percent)	1.84%
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	$ 81
Held-to-maturity yield after one but within five years (percent)	3.61%
Held-to-maturity after five but within ten years	$ 780
Held-to-maturity yield after five but within ten years (percent)	3.45%
Held-to-maturity after ten years	$ 391
Held-to-maturity yield after ten years (percent)	2.84%
U.S. Government agency issued or guaranteed
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	$ 68
Available-for-sale yield after one but within five years (percent)	3.21%
Available-for-sale amount after five but within ten years	$ 2
Available-for-sale yield after five but within ten years (percent)	5.52%
Available-for-sale amount after ten years	$ 8,594
Available-for-sale yield after ten years (percent)	2.99%
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)	5.39%
Held-to-maturity after five but within ten years	$ 14
Held-to-maturity yield after five but within ten years (percent)	5.74%
Held-to-maturity after ten years	$ 12,011
Held-to-maturity yield after ten years (percent)	4.50%
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)	3.97%
Held-to-maturity amount after one but within five years	$ 2
Held-to-maturity yield after one but within five years (percent)	4.11%
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	$ 55
Available-for-sale yield after one but within five years (percent)	4.36%
Available-for-sale amount after five but within ten years	$ 48
Available-for-sale yield after five but within ten years (percent)	3.88%
Available-for-sale amount after ten years	$ 12
Available-for-sale yield after ten years (percent)	5.78%
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	$ 1
Held-to-maturity yield after five but within ten years (percent)	4.19%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years (percent)	0.00%
Foreign debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,383
Available-for-sale yield within one year (percent)	0.39%
Available-for-sale amount after one but within five years	$ 1,811
Available-for-sale yield after one but within five years (percent)	4.54%
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%