Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,883
Available-for-sale yield within one year	1.64%
Available-for-sale amount after one but within five years	$ 8,798
Available-for-sale yield after one but within five years	2.25%
Available-for-sale amount after five but within ten years	$ 7,473
Available-for-sale yield after five but within ten years	2.38%
Available-for-sale amount after ten years	$ 13,239
Available-for-sale yield after ten years	2.89%
Total fair value within one year	$ 2,887
Total fair value after one but within five years	8,771
Total fair value after five but within ten years	7,301
Total fair value after ten years	13,081
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	$ 397
Held-to-maturity yield after one but within five years	2.76%
Held-to-maturity after five but within ten years	$ 190
Held-to-maturity yield after five but within ten years	2.79%
Held-to-maturity after ten years	$ 13,745
Held-to-maturity yield after ten years	2.74%
Total fair value within one year	$ 51
Total fair value after one but within five years	398
Total fair value after five but within ten years	189
Total fair value after ten years	13,661
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 298
Available-for-sale yield within one year	2.58%
Available-for-sale amount after one but within five years	$ 5,761
Available-for-sale yield after one but within five years	2.02%
Available-for-sale amount after five but within ten years	$ 4,985
Available-for-sale yield after five but within ten years	2.14%
Available-for-sale amount after ten years	$ 2,823
Available-for-sale yield after ten years	2.96%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,187
Available-for-sale yield within one year	3.15%
Available-for-sale amount after one but within five years	$ 1,567
Available-for-sale yield after one but within five years	2.75%
Available-for-sale amount after five but within ten years	$ 2,424
Available-for-sale yield after five but within ten years	2.80%
Available-for-sale amount after ten years	$ 3,832
Available-for-sale yield after ten years	3.09%
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	$ 378
Held-to-maturity yield after one but within five years	2.71%
Held-to-maturity after five but within ten years	$ 160
Held-to-maturity yield after five but within ten years	2.63%
Held-to-maturity after ten years	$ 3,128
Held-to-maturity yield after ten years	3.00%
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	$ 74
Available-for-sale yield after one but within five years	2.16%
Available-for-sale amount after five but within ten years	$ 7
Available-for-sale yield after five but within ten years	4.17%
Available-for-sale amount after ten years	$ 6,495
Available-for-sale yield after ten years	2.73%
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	$ 14
Held-to-maturity yield after one but within five years	3.94%
Held-to-maturity after five but within ten years	$ 25
Held-to-maturity yield after five but within ten years	3.56%
Held-to-maturity after ten years	$ 10,615
Held-to-maturity yield after ten years	2.66%
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	4.71%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.44%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.19%
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	$ 57
Available-for-sale yield after five but within ten years	5.31%
Available-for-sale amount after ten years	$ 89
Available-for-sale yield after ten years	4.02%
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	8.38%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,398
Available-for-sale yield within one year	0.16%
Available-for-sale amount after one but within five years	$ 1,396
Available-for-sale yield after one but within five years	2.62%
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%