Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,138
Available-for-sale yield within one year	1.40%
Available-for-sale amount after one but within five years	$ 8,339
Available-for-sale yield after one but within five years	2.21%
Available-for-sale amount after five but within ten years	$ 8,011
Available-for-sale yield after five but within ten years	2.39%
Available-for-sale amount after ten years	$ 12,472
Available-for-sale yield after ten years	2.96%
Total fair value within one year	$ 3,143
Total fair value after one but within five years	8,288
Total fair value after five but within ten years	7,746
Total fair value after ten years	12,202
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	$ 412
Held-to-maturity yield after one but within five years	2.77%
Held-to-maturity after five but within ten years	$ 194
Held-to-maturity yield after five but within ten years	2.80%
Held-to-maturity after ten years	$ 14,013
Held-to-maturity yield after ten years	2.73%
Total fair value within one year	$ 51
Total fair value after one but within five years	409
Total fair value after five but within ten years	190
Total fair value after ten years	13,793
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	$ 5,456
Available-for-sale yield after one but within five years	2.03%
Available-for-sale amount after five but within ten years	$ 5,701
Available-for-sale yield after five but within ten years	2.19%
Available-for-sale amount after ten years	$ 2,392
Available-for-sale yield after ten years	3.10%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,358
Available-for-sale yield within one year	3.19%
Available-for-sale amount after one but within five years	$ 1,261
Available-for-sale yield after one but within five years	2.64%
Available-for-sale amount after five but within ten years	$ 2,244
Available-for-sale yield after five but within ten years	2.81%
Available-for-sale amount after ten years	$ 4,195
Available-for-sale yield after ten years	3.16%
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	$ 389
Held-to-maturity yield after one but within five years	2.71%
Held-to-maturity after five but within ten years	$ 163
Held-to-maturity yield after five but within ten years	2.63%
Held-to-maturity after ten years	$ 3,088
Held-to-maturity yield after ten years	2.96%
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	$ 72
Available-for-sale yield after one but within five years	2.17%
Available-for-sale amount after five but within ten years	$ 10
Available-for-sale yield after five but within ten years	3.65%
Available-for-sale amount after ten years	$ 5,795
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	$ 18
Held-to-maturity yield after one but within five years	3.86%
Held-to-maturity after five but within ten years	$ 26
Held-to-maturity yield after five but within ten years	3.58%
Held-to-maturity after ten years	$ 10,923
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	4.67%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.17%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.37%
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	$ 56
Available-for-sale yield after five but within ten years	5.42%
Available-for-sale amount after ten years	$ 90
Available-for-sale yield after ten years	4.03%
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	$ 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	8.41%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,780
Available-for-sale yield within one year	0.04%
Available-for-sale amount after one but within five years	$ 1,550
Available-for-sale yield after one but within five years	2.48%
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%