Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 4,852
Available-for-sale yield within one year	0.90%
Available-for-sale amount after one but within five years	$ 7,008
Available-for-sale yield after one but within five years	1.12%
Available-for-sale amount after five but within ten years	$ 9,250
Available-for-sale yield after five but within ten years	1.76%
Available-for-sale amount after ten years	$ 18,526
Available-for-sale yield after ten years	1.96%
Total fair value within one year	$ 4,879
Total fair value after one but within five years	7,161
Total fair value after five but within ten years	9,709
Total fair value after ten years	18,923
Held-to-maturity amount within one year	$ 111
Held-to-maturity yield within one year	2.49%
Held-to-maturity amount after one but within five years	$ 273
Held-to-maturity yield after one but within five years	2.76%
Held-to-maturity after five but within ten years	$ 452
Held-to-maturity yield after five but within ten years	2.34%
Held-to-maturity after ten years	$ 8,147
Held-to-maturity yield after ten years	2.67%
Total fair value within one year	$ 111
Total fair value after one but within five years	283
Total fair value after five but within ten years	467
Total fair value after ten years	8,508
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,026
Available-for-sale yield within one year	3.27%
Available-for-sale amount after one but within five years	$ 4,777
Available-for-sale yield after one but within five years	1.00%
Available-for-sale amount after five but within ten years	$ 6,090
Available-for-sale yield after five but within ten years	1.39%
Available-for-sale amount after ten years	$ 4,194
Available-for-sale yield after ten years	2.00%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 30
Available-for-sale yield within one year	2.78%
Available-for-sale amount after one but within five years	$ 803
Available-for-sale yield after one but within five years	2.12%
Available-for-sale amount after five but within ten years	$ 3,098
Available-for-sale yield after five but within ten years	2.43%
Available-for-sale amount after ten years	$ 4,967
Available-for-sale yield after ten years	1.78%
Held-to-maturity amount within one year	$ 110
Held-to-maturity yield within one year	2.49%
Held-to-maturity amount after one but within five years	$ 258
Held-to-maturity yield after one but within five years	2.72%
Held-to-maturity after five but within ten years	$ 436
Held-to-maturity yield after five but within ten years	2.25%
Held-to-maturity after ten years	$ 1,113
Held-to-maturity yield after ten years	3.25%
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	$ 97
Available-for-sale yield after one but within five years	1.81%
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	$ 9,284
Available-for-sale yield after ten years	2.02%
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	$ 9
Held-to-maturity yield after one but within five years	3.79%
Held-to-maturity after five but within ten years	$ 13
Held-to-maturity yield after five but within ten years	4.51%
Held-to-maturity after ten years	$ 7,032
Held-to-maturity yield after ten years	2.57%
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	2.69%
Held-to-maturity amount after one but within five years	$ 6
Held-to-maturity yield after one but within five years	3.20%
Held-to-maturity after five but within ten years	$ 3
Held-to-maturity yield after five but within ten years	4.93%
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	$ 61
Available-for-sale yield after five but within ten years	4.90%
Available-for-sale amount after ten years	$ 81
Available-for-sale yield after ten years	3.14%
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	5.95%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,796
Available-for-sale yield within one year	0.24%
Available-for-sale amount after one but within five years	$ 1,331
Available-for-sale yield after one but within five years	0.93%
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%