Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Jun. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 4,451
Available-for-sale yield within one year	0.87%
Available-for-sale amount after one but within five years	$ 6,958
Available-for-sale yield after one but within five years	1.49%
Available-for-sale amount after five but within ten years	$ 9,727
Available-for-sale yield after five but within ten years	1.78%
Available-for-sale amount after ten years	$ 19,468
Available-for-sale yield after ten years	2.22%
Total fair value within one year	$ 4,458
Total fair value after one but within five years	7,127
Total fair value after five but within ten years	10,277
Total fair value after ten years	19,806
Held-to-maturity amount within one year	$ 18
Held-to-maturity yield within one year	2.74%
Held-to-maturity amount after one but within five years	$ 463
Held-to-maturity yield after one but within five years	2.70%
Held-to-maturity after five but within ten years	$ 551
Held-to-maturity yield after five but within ten years	2.35%
Held-to-maturity after ten years	$ 10,605
Held-to-maturity yield after ten years	2.71%
Total fair value within one year	$ 18
Total fair value after one but within five years	477
Total fair value after five but within ten years	569
Total fair value after ten years	11,042
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,020
Available-for-sale yield within one year	0.43%
Available-for-sale amount after one but within five years	$ 4,687
Available-for-sale yield after one but within five years	1.50%
Available-for-sale amount after five but within ten years	$ 6,579
Available-for-sale yield after five but within ten years	1.38%
Available-for-sale amount after ten years	$ 5,409
Available-for-sale yield after ten years	1.80%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 735
Available-for-sale yield within one year	2.82%
Available-for-sale amount after one but within five years	$ 663
Available-for-sale yield after one but within five years	1.92%
Available-for-sale amount after five but within ten years	$ 3,085
Available-for-sale yield after five but within ten years	2.55%
Available-for-sale amount after ten years	$ 2,902
Available-for-sale yield after ten years	2.37%
Held-to-maturity amount within one year	$ 18
Held-to-maturity yield within one year	2.73%
Held-to-maturity amount after one but within five years	$ 445
Held-to-maturity yield after one but within five years	2.66%
Held-to-maturity after five but within ten years	$ 531
Held-to-maturity yield after five but within ten years	2.27%
Held-to-maturity after ten years	$ 1,554
Held-to-maturity yield after ten years	3.21%
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	$ 99
Available-for-sale yield after one but within five years	1.84%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years	7.88%
Available-for-sale amount after ten years	$ 11,072
Available-for-sale yield after ten years	2.38%
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	$ 12
Held-to-maturity yield after one but within five years	3.76%
Held-to-maturity after five but within ten years	$ 16
Held-to-maturity yield after five but within ten years	4.60%
Held-to-maturity after ten years	$ 9,049
Held-to-maturity yield after ten years	2.62%
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
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	$ 6
Held-to-maturity yield after one but within five years	3.56%
Held-to-maturity after five but within ten years	$ 4
Held-to-maturity yield after five but within ten years	4.17%
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	$ 62
Available-for-sale yield after five but within ten years	4.84%
Available-for-sale amount after ten years	$ 85
Available-for-sale yield after ten years	3.02%
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	5.94%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,696
Available-for-sale yield within one year	0.51%
Available-for-sale amount after one but within five years	$ 1,509
Available-for-sale yield after one but within five years	1.24%
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%