Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,757
Available-for-sale yield within one year	1.11%
Available-for-sale amount after one but within five years	$ 7,192
Available-for-sale yield after one but within five years	1.96%
Available-for-sale amount after five but within ten years	$ 9,210
Available-for-sale yield after five but within ten years	1.86%
Available-for-sale amount after ten years	$ 16,573
Available-for-sale yield after ten years	2.67%
Total fair value within one year	$ 2,762
Total fair value after one but within five years	7,234
Total fair value after five but within ten years	9,122
Total fair value after ten years	16,545
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	$ 501
Held-to-maturity yield after one but within five years	2.69%
Held-to-maturity after five but within ten years	$ 574
Held-to-maturity yield after five but within ten years	2.35%
Held-to-maturity after ten years	$ 12,218
Held-to-maturity yield after ten years	2.72%
Total fair value within one year	$ 0
Total fair value after one but within five years	506
Total fair value after five but within ten years	577
Total fair value after ten years	12,348
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	$ 5,695
Available-for-sale yield after one but within five years	1.87%
Available-for-sale amount after five but within ten years	$ 7,199
Available-for-sale yield after five but within ten years	1.60%
Available-for-sale amount after ten years	$ 3,325
Available-for-sale yield after ten years	2.62%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 725
Available-for-sale yield within one year	2.82%
Available-for-sale amount after one but within five years	$ 168
Available-for-sale yield after one but within five years	3.12%
Available-for-sale amount after five but within ten years	$ 1,953
Available-for-sale yield after five but within ten years	2.74%
Available-for-sale amount after ten years	$ 2,239
Available-for-sale yield after ten years	2.90%
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	$ 482
Held-to-maturity yield after one but within five years	2.66%
Held-to-maturity after five but within ten years	$ 551
Held-to-maturity yield after five but within ten years	2.30%
Held-to-maturity after ten years	$ 2,017
Held-to-maturity yield after ten years	3.16%
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	$ 79
Available-for-sale yield after one but within five years	2.18%
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	$ 10,925
Available-for-sale yield after ten years	2.63%
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	3.74%
Held-to-maturity after five but within ten years	$ 19
Held-to-maturity yield after five but within ten years	3.48%
Held-to-maturity after ten years	$ 10,199
Held-to-maturity yield after ten years	2.63%
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	3.50%
Held-to-maturity after five but within ten years	$ 4
Held-to-maturity yield after five but within ten years	4.45%
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	5.19%
Available-for-sale amount after ten years	$ 84
Available-for-sale yield after ten years	3.71%
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	7.50%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 2,032
Available-for-sale yield within one year	0.50%
Available-for-sale amount after one but within five years	$ 1,250
Available-for-sale yield after one but within five years	2.17%
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%