Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 476
Available-for-sale yield within one year	1.89%
Available-for-sale amount after one but within five years	$ 10,498
Available-for-sale yield after one but within five years	2.01%
Available-for-sale amount after five but within ten years	$ 11,292
Available-for-sale yield after five but within ten years	2.02%
Available-for-sale amount after ten years	$ 15,077
Available-for-sale yield after ten years	2.48%
Total fair value within one year	$ 478
Total fair value after one but within five years	10,627
Total fair value after five but within ten years	10,978
Total fair value after ten years	14,673
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year	3.83%
Held-to-maturity amount after one but within five years	$ 308
Held-to-maturity yield after one but within five years	2.16%
Held-to-maturity after five but within ten years	$ 434
Held-to-maturity yield after five but within ten years	2.83%
Held-to-maturity after ten years	$ 12,064
Held-to-maturity yield after ten years	2.44%
Total fair value within one year	$ 3
Total fair value after one but within five years	309
Total fair value after five but within ten years	436
Total fair value after ten years	12,090
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	$ 7,169
Available-for-sale yield after one but within five years	1.66%
Available-for-sale amount after five but within ten years	$ 10,018
Available-for-sale yield after five but within ten years	1.97%
Available-for-sale amount after ten years	$ 4,179
Available-for-sale yield after ten years	2.89%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 200
Available-for-sale yield within one year	3.25%
Available-for-sale amount after one but within five years	$ 3,146
Available-for-sale yield after one but within five years	2.81%
Available-for-sale amount after five but within ten years	$ 1,180
Available-for-sale yield after five but within ten years	2.41%
Available-for-sale amount after ten years	$ 5,857
Available-for-sale yield after ten years	2.32%
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	$ 299
Held-to-maturity yield after one but within five years	2.15%
Held-to-maturity after five but within ten years	$ 393
Held-to-maturity yield after five but within ten years	2.78%
Held-to-maturity after ten years	$ 3,364
Held-to-maturity yield after ten years	2.85%
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	$ 0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 116
Available-for-sale yield after one but within five years	2.92%
Available-for-sale amount after five but within ten years	$ 94
Available-for-sale yield after five but within ten years	2.49%
Available-for-sale amount after ten years	$ 4,685
Available-for-sale yield after ten years	2.31%
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	1.91%
Held-to-maturity after five but within ten years	$ 36
Held-to-maturity yield after five but within ten years	3.28%
Held-to-maturity after ten years	$ 8,691
Held-to-maturity yield after ten years	2.28%
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	$ 3
Held-to-maturity yield within one year	3.83%
Held-to-maturity amount after one but within five years	$ 3
Held-to-maturity yield after one but within five years	3.74%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.33%
Held-to-maturity after ten years	$ 4
Held-to-maturity yield after ten years	5.63%
Asset backed securities [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	$ 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	$ 177
Available-for-sale yield after ten years	3.48%
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	$ 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	$ 5
Held-to-maturity yield after ten years	7.03%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 276
Available-for-sale yield within one year	0.90%
Available-for-sale amount after one but within five years	$ 67
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	$ 179
Available-for-sale yield after ten years	1.29%