Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,798
Available-for-sale yield within one year	1.27%
Available-for-sale amount after one but within five years	$ 10,428
Available-for-sale yield after one but within five years	2.03%
Available-for-sale amount after five but within ten years	$ 8,244
Available-for-sale yield after five but within ten years	2.26%
Available-for-sale amount after ten years	$ 15,852
Available-for-sale yield after ten years	2.81%
Total fair value within one year	$ 2,801
Total fair value after one but within five years	10,445
Total fair value after five but within ten years	8,191
Total fair value after ten years	15,817
Held-to-maturity amount within one year	$ 51
Held-to-maturity yield within one year	2.27%
Held-to-maturity amount after one but within five years	$ 394
Held-to-maturity yield after one but within five years	2.77%
Held-to-maturity after five but within ten years	$ 296
Held-to-maturity yield after five but within ten years	2.57%
Held-to-maturity after ten years	$ 13,186
Held-to-maturity yield after ten years	2.76%
Total fair value within one year	$ 51
Total fair value after one but within five years	399
Total fair value after five but within ten years	298
Total fair value after ten years	13,263
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 199
Available-for-sale yield within one year	2.22%
Available-for-sale amount after one but within five years	$ 7,760
Available-for-sale yield after one but within five years	1.81%
Available-for-sale amount after five but within ten years	$ 5,967
Available-for-sale yield after five but within ten years	2.05%
Available-for-sale amount after ten years	$ 3,016
Available-for-sale yield after ten years	2.80%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,047
Available-for-sale yield within one year	2.99%
Available-for-sale amount after one but within five years	$ 1,168
Available-for-sale yield after one but within five years	2.83%
Available-for-sale amount after five but within ten years	$ 2,218
Available-for-sale yield after five but within ten years	2.74%
Available-for-sale amount after ten years	$ 3,779
Available-for-sale yield after ten years	3.00%
Held-to-maturity amount within one year	$ 50
Held-to-maturity yield within one year	2.22%
Held-to-maturity amount after one but within five years	$ 376
Held-to-maturity yield after one but within five years	2.72%
Held-to-maturity after five but within ten years	$ 269
Held-to-maturity yield after five but within ten years	2.45%
Held-to-maturity after ten years	$ 2,866
Held-to-maturity yield after ten years	3.04%
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	$ 81
Available-for-sale yield after one but within five years	2.21%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years	4.33%
Available-for-sale amount after ten years	$ 8,968
Available-for-sale yield after ten years	2.72%
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	$ 13
Held-to-maturity yield after one but within five years	3.94%
Held-to-maturity after five but within ten years	$ 22
Held-to-maturity yield after five but within ten years	3.69%
Held-to-maturity after ten years	$ 10,318
Held-to-maturity yield after ten years	2.68%
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year	4.75%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.46%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.20%
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	$ 58
Available-for-sale yield after five but within ten years	5.21%
Available-for-sale amount after ten years	$ 89
Available-for-sale yield after ten years	3.90%
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	7.83%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,552
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 1,419
Available-for-sale yield after one but within five years	2.56%
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%