Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,820
Available-for-sale yield within one year	0.93%
Available-for-sale amount after one but within five years	$ 7,186
Available-for-sale yield after one but within five years	2.34%
Available-for-sale amount after five but within ten years	$ 9,813
Available-for-sale yield after five but within ten years	2.27%
Available-for-sale amount after ten years	$ 12,851
Available-for-sale yield after ten years	2.91%
Total fair value within one year	$ 1,824
Total fair value after one but within five years	7,171
Total fair value after five but within ten years	9,443
Total fair value after ten years	12,590
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year	4.29%
Held-to-maturity amount after one but within five years	$ 455
Held-to-maturity yield after one but within five years	2.70%
Held-to-maturity after five but within ten years	$ 206
Held-to-maturity yield after five but within ten years	2.91%
Held-to-maturity after ten years	$ 14,490
Held-to-maturity yield after ten years	2.63%
Total fair value within one year	$ 1
Total fair value after one but within five years	449
Total fair value after five but within ten years	201
Total fair value after ten years	14,154
U.S. Treasury [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	$ 3,979
Available-for-sale yield after one but within five years	2.16%
Available-for-sale amount after five but within ten years	$ 7,342
Available-for-sale yield after five but within ten years	2.11%
Available-for-sale amount after ten years	$ 2,698
Available-for-sale yield after ten years	3.13%
US Government-sponsored Enterprises Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 370
Available-for-sale yield within one year	3.83%
Available-for-sale amount after one but within five years	$ 2,147
Available-for-sale yield after one but within five years	2.94%
Available-for-sale amount after five but within ten years	$ 2,460
Available-for-sale yield after five but within ten years	2.76%
Available-for-sale amount after ten years	$ 3,908
Available-for-sale yield after ten years	3.08%
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	$ 430
Held-to-maturity yield after one but within five years	2.64%
Held-to-maturity after five but within ten years	$ 171
Held-to-maturity yield after five but within ten years	2.75%
Held-to-maturity after ten years	$ 3,115
Held-to-maturity yield after ten years	2.89%
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 50
Available-for-sale yield within one year	1.85%
Available-for-sale amount after one but within five years	$ 72
Available-for-sale yield after one but within five years	2.23%
Available-for-sale amount after five but within ten years	$ 11
Available-for-sale yield after five but within ten years	3.89%
Available-for-sale amount after ten years	$ 6,095
Available-for-sale yield after ten years	2.67%
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	$ 20
Held-to-maturity yield after one but within five years	3.72%
Held-to-maturity after five but within ten years	$ 30
Held-to-maturity yield after five but within ten years	3.51%
Held-to-maturity after ten years	$ 11,372
Held-to-maturity yield after ten years	2.56%
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	4.29%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.64%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.72%
Held-to-maturity after ten years	$ 1
Held-to-maturity yield after ten years	4.41%
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	$ 0
Available-for-sale yield after five but within ten years	0.00%
Available-for-sale amount after ten years	$ 150
Available-for-sale yield after ten years	4.41%
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	8.84%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,400
Available-for-sale yield within one year	0.13%
Available-for-sale amount after one but within five years	$ 988
Available-for-sale yield after one but within five years	1.80%
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%