Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 1,277
Available-for-sale yield within one year	1.38%
Available-for-sale amount after one but within five years	$ 6,986
Available-for-sale yield after one but within five years	2.49%
Available-for-sale amount after five but within ten years	$ 9,929
Available-for-sale yield after five but within ten years	2.10%
Available-for-sale amount after ten years	$ 12,606
Available-for-sale yield after ten years	2.66%
Total fair value within one year	$ 1,278
Total fair value after one but within five years	7,060
Total fair value after five but within ten years	9,706
Total fair value after ten years	12,479
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	$ 347
Held-to-maturity yield after one but within five years	2.51%
Held-to-maturity after five but within ten years	$ 358
Held-to-maturity yield after five but within ten years	2.78%
Held-to-maturity after ten years	$ 13,271
Held-to-maturity yield after ten years	2.48%
Total fair value within one year	$ 1
Total fair value after one but within five years	349
Total fair value after five but within ten years	359
Total fair value after ten years	13,193
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [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	$ 3,878
Available-for-sale yield after one but within five years	2.14%
Available-for-sale amount after five but within ten years	$ 7,939
Available-for-sale yield after five but within ten years	1.97%
Available-for-sale amount after ten years	$ 3,523
Available-for-sale yield after ten years	2.88%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 148
Available-for-sale yield within one year	3.52%
Available-for-sale amount after one but within five years	$ 2,754
Available-for-sale yield after one but within five years	3.04%
Available-for-sale amount after five but within ten years	$ 1,972
Available-for-sale yield after five but within ten years	2.61%
Available-for-sale amount after ten years	$ 2,759
Available-for-sale yield after ten years	2.70%
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	$ 322
Held-to-maturity yield after one but within five years	2.39%
Held-to-maturity after five but within ten years	$ 325
Held-to-maturity yield after five but within ten years	2.78%
Held-to-maturity after ten years	$ 2,747
Held-to-maturity yield after ten years	2.86%
US Government agency issued or guaranteed [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 50
Available-for-sale yield within one year	1.89%
Available-for-sale amount after one but within five years	$ 131
Available-for-sale yield after one but within five years	2.14%
Available-for-sale amount after five but within ten years	$ 18
Available-for-sale yield after five but within ten years	3.98%
Available-for-sale amount after ten years	$ 6,164
Available-for-sale yield after ten years	2.48%
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	$ 21
Held-to-maturity yield after one but within five years	3.99%
Held-to-maturity after five but within ten years	$ 27
Held-to-maturity yield after five but within ten years	2.53%
Held-to-maturity after ten years	$ 10,520
Held-to-maturity yield after ten years	2.38%
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [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	$ 4
Held-to-maturity yield after one but within five years	4.24%
Held-to-maturity after five but within ten years	$ 6
Held-to-maturity yield after five but within ten years	4.26%
Held-to-maturity after ten years	$ 1
Held-to-maturity yield after ten years	4.41%
Asset backed securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 400
Available-for-sale yield within one year	2.64%
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	$ 160
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	$ 3
Held-to-maturity yield after ten years	6.57%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 679
Available-for-sale yield within one year	0.13%
Available-for-sale amount after one but within five years	$ 223
Available-for-sale yield after one but within five years	1.93%
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%