Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,027
Available-for-sale yield within one year	1.15%
Available-for-sale amount after one but within five years	$ 7,894
Available-for-sale yield after one but within five years	1.39%
Available-for-sale amount after five but within ten years	$ 8,690
Available-for-sale yield after five but within ten years	1.93%
Available-for-sale amount after ten years	$ 20,256
Available-for-sale yield after ten years	2.14%
Total fair value within one year	$ 3,031
Total fair value after one but within five years	7,997
Total fair value after five but within ten years	9,176
Total fair value after ten years	20,588
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	$ 498
Held-to-maturity yield after one but within five years	2.70%
Held-to-maturity after five but within ten years	$ 592
Held-to-maturity yield after five but within ten years	2.34%
Held-to-maturity after ten years	$ 11,893
Held-to-maturity yield after ten years	2.67%
Total fair value within one year	$ 0
Total fair value after one but within five years	512
Total fair value after five but within ten years	607
Total fair value after ten years	12,327
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 55
Available-for-sale yield within one year	2.93%
Available-for-sale amount after one but within five years	$ 5,308
Available-for-sale yield after one but within five years	1.27%
Available-for-sale amount after five but within ten years	$ 6,201
Available-for-sale yield after five but within ten years	1.61%
Available-for-sale amount after ten years	$ 5,329
Available-for-sale yield after ten years	2.10%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 730
Available-for-sale yield within one year	2.82%
Available-for-sale amount after one but within five years	$ 516
Available-for-sale yield after one but within five years	1.90%
Available-for-sale amount after five but within ten years	$ 2,426
Available-for-sale yield after five but within ten years	2.66%
Available-for-sale amount after ten years	$ 3,136
Available-for-sale yield after ten years	2.55%
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	$ 480
Held-to-maturity yield after one but within five years	2.66%
Held-to-maturity after five but within ten years	$ 568
Held-to-maturity yield after five but within ten years	2.26%
Held-to-maturity after ten years	$ 1,802
Held-to-maturity yield after ten years	3.19%
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	$ 80
Available-for-sale yield after one but within five years	2.12%
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	$ 11,705
Available-for-sale yield after ten years	2.03%
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	3.92%
Held-to-maturity after five but within ten years	$ 20
Held-to-maturity yield after five but within ten years	4.18%
Held-to-maturity after ten years	$ 10,089
Held-to-maturity yield after ten years	2.57%
Obligations of U.S. states and political subdivision [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	3.58%
Held-to-maturity after five but within ten years	$ 4
Held-to-maturity yield after five but within ten years	4.16%
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	4.85%
Available-for-sale amount after ten years	$ 86
Available-for-sale yield after ten years	3.19%
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	6.02%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,242
Available-for-sale yield within one year	0.56%
Available-for-sale amount after one but within five years	$ 1,990
Available-for-sale yield after one but within five years	1.52%
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%