Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Jun. 30, 2017 USD ($)
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	1.96%
Available-for-sale amount after one but within five years	$ 8,134
Available-for-sale yield after one but within five years	2.33%
Available-for-sale amount after five but within ten years	$ 10,432
Available-for-sale yield after five but within ten years	2.03%
Available-for-sale amount after ten years	$ 13,549
Available-for-sale yield after ten years	2.80%
Total fair value within one year	$ 301
Total fair value after one but within five years	8,260
Total fair value after five but within ten years	10,245
Total fair value after ten years	13,355
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.04%
Held-to-maturity amount after one but within five years	$ 334
Held-to-maturity yield after one but within five years	2.46%
Held-to-maturity after five but within ten years	$ 367
Held-to-maturity yield after five but within ten years	2.76%
Held-to-maturity after ten years	$ 12,731
Held-to-maturity yield after ten years	2.45%
Total fair value within one year	$ 2
Total fair value after one but within five years	337
Total fair value after five but within ten years	371
Total fair value after ten years	12,768
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	$ 4,209
Available-for-sale yield after one but within five years	1.98%
Available-for-sale amount after five but within ten years	$ 8,821
Available-for-sale yield after five but within ten years	1.94%
Available-for-sale amount after ten years	$ 3,841
Available-for-sale yield after ten years	2.87%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 178
Available-for-sale yield within one year	3.26%
Available-for-sale amount after one but within five years	$ 3,118
Available-for-sale yield after one but within five years	2.89%
Available-for-sale amount after five but within ten years	$ 1,520
Available-for-sale yield after five but within ten years	2.52%
Available-for-sale amount after ten years	$ 3,499
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	$ 298
Held-to-maturity yield after one but within five years	2.32%
Held-to-maturity after five but within ten years	$ 356
Held-to-maturity yield after five but within ten years	2.76%
Held-to-maturity after ten years	$ 3,047
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	2.90%
Available-for-sale amount after five but within ten years	$ 91
Available-for-sale yield after five but within ten years	2.42%
Available-for-sale amount after ten years	$ 6,040
Available-for-sale yield after ten years	2.77%
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	$ 33
Held-to-maturity yield after one but within five years	3.64%
Held-to-maturity after five but within ten years	$ 6
Held-to-maturity yield after five but within ten years	2.15%
Held-to-maturity after ten years	$ 9,675
Held-to-maturity yield after ten years	2.32%
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	4.04%
Held-to-maturity amount after one but within five years	$ 3
Held-to-maturity yield after one but within five years	3.77%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.40%
Held-to-maturity after ten years	$ 4
Held-to-maturity yield after ten years	5.66%
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	$ 169
Available-for-sale yield after ten years	3.76%
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	6.69%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 121
Available-for-sale yield within one year	0.02%
Available-for-sale amount after one but within five years	$ 292
Available-for-sale yield after one but within five years	1.23%
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%