Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 361
Available-for-sale yield within one year	2.70%
Available-for-sale amount after one but within five years	$ 12,422
Available-for-sale yield after one but within five years	2.00%
Available-for-sale amount after five but within ten years	$ 10,927
Available-for-sale yield after five but within ten years	2.02%
Available-for-sale amount after ten years	$ 14,469
Available-for-sale yield after ten years	2.63%
Total fair value within one year	$ 363
Total fair value after one but within five years	12,552
Total fair value after five but within ten years	10,682
Total fair value after ten years	14,160
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	3.95%
Held-to-maturity amount after one but within five years	$ 307
Held-to-maturity yield after one but within five years	2.19%
Held-to-maturity after five but within ten years	$ 431
Held-to-maturity yield after five but within ten years	2.85%
Held-to-maturity after ten years	$ 12,575
Held-to-maturity yield after ten years	2.47%
Total fair value within one year	$ 2
Total fair value after one but within five years	308
Total fair value after five but within ten years	435
Total fair value after ten years	12,588
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	$ 8,566
Available-for-sale yield after one but within five years	1.67%
Available-for-sale amount after five but within ten years	$ 9,670
Available-for-sale yield after five but within ten years	1.96%
Available-for-sale amount after ten years	$ 4,135
Available-for-sale yield after ten years	2.93%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 299
Available-for-sale yield within one year	3.25%
Available-for-sale amount after one but within five years	$ 3,096
Available-for-sale yield after one but within five years	2.89%
Available-for-sale amount after five but within ten years	$ 1,165
Available-for-sale yield after five but within ten years	2.48%
Available-for-sale amount after ten years	$ 5,864
Available-for-sale yield after ten years	2.57%
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	2.17%
Held-to-maturity after five but within ten years	$ 391
Held-to-maturity yield after five but within ten years	2.78%
Held-to-maturity after ten years	$ 3,201
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	$ 115
Available-for-sale yield after one but within five years	2.93%
Available-for-sale amount after five but within ten years	$ 92
Available-for-sale yield after five but within ten years	2.47%
Available-for-sale amount after ten years	$ 4,297
Available-for-sale yield after ten years	2.39%
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	$ 5
Held-to-maturity yield after one but within five years	1.94%
Held-to-maturity after five but within ten years	$ 35
Held-to-maturity yield after five but within ten years	3.51%
Held-to-maturity after ten years	$ 9,365
Held-to-maturity yield after ten years	2.33%
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	$ 2
Held-to-maturity yield within one year	3.95%
Held-to-maturity amount after one but within five years	$ 3
Held-to-maturity yield after one but within five years	3.75%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.37%
Held-to-maturity after ten years	$ 4
Held-to-maturity yield after ten years	5.64%
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	$ 400
Available-for-sale yield after one but within five years	2.38%
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	$ 173
Available-for-sale yield after ten years	3.62%
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	6.85%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 62
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 245
Available-for-sale yield after one but within five years	0.93%
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%