Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) - Jun. 30, 2015 - USD ($) $ in Millions	Total
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 406
Available-for-sale yield within one year	2.35%
Available-for-sale amount after one but within five years	$ 8,798
Available-for-sale yield after one but within five years	1.56%
Available-for-sale amount after five but within ten years	$ 6,351
Available-for-sale yield after five but within ten years	2.38%
Available-for-sale amount after ten years	$ 17,285
Available-for-sale yield after ten years	2.83%
Total fair value within one year	$ 406
Total fair value after one but within five years	8,896
Total fair value after five but within ten years	6,471
Total fair value after ten years	17,149
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.76%
Held-to-maturity amount after one but within five years	$ 101
Held-to-maturity yield after one but within five years	1.65%
Held-to-maturity after five but within ten years	$ 663
Held-to-maturity yield after five but within ten years	2.65%
Held-to-maturity after ten years	$ 13,394
Held-to-maturity yield after ten years	2.55%
Total fair value within one year	$ 2
Total fair value after one but within five years	102
Total fair value after five but within ten years	669
Total fair value after ten years	13,554
U.S. Treasury [Member]
Schedule of Available-for-sale 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	$ 3,357
Available-for-sale yield after one but within five years	0.76%
Available-for-sale amount after five but within ten years	$ 4,254
Available-for-sale yield after five but within ten years	2.25%
Available-for-sale amount after ten years	$ 3,127
Available-for-sale yield after ten years	3.22%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale 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	$ 2,066
Available-for-sale yield after one but within five years	3.12%
Available-for-sale amount after five but within ten years	$ 1,790
Available-for-sale yield after five but within ten years	2.60%
Available-for-sale amount after ten years	$ 1,042
Available-for-sale yield after ten years	3.02%
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	$ 92
Held-to-maturity yield after one but within five years	1.39%
Held-to-maturity after five but within ten years	$ 612
Held-to-maturity yield after five but within ten years	2.62%
Held-to-maturity after ten years	$ 4,355
Held-to-maturity yield after ten years	2.96%
US Government agency issued or guaranteed [Member]
Schedule of Available-for-sale 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
Available-for-sale yield after one but within five years	4.21%
Available-for-sale amount after five but within ten years	$ 39
Available-for-sale yield after five but within ten years	3.89%
Available-for-sale amount after ten years	$ 12,411
Available-for-sale yield after ten years	2.68%
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	$ 1
Held-to-maturity yield after one but within five years	7.71%
Held-to-maturity after five but within ten years	$ 45
Held-to-maturity yield after five but within ten years	2.91%
Held-to-maturity after ten years	$ 9,026
Held-to-maturity yield after ten years	2.35%
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale 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	$ 71
Available-for-sale yield after one but within five years	4.17%
Available-for-sale amount after five but within ten years	$ 268
Available-for-sale yield after five but within ten years	2.82%
Available-for-sale amount after ten years	$ 479
Available-for-sale yield after ten years	3.40%
Held-to-maturity amount within one year	$ 2
Held-to-maturity yield within one year	4.76%
Held-to-maturity amount after one but within five years	$ 8
Held-to-maturity yield after one but within five years	3.96%
Held-to-maturity after five but within ten years	$ 6
Held-to-maturity yield after five but within ten years	3.82%
Held-to-maturity after ten years	$ 4
Held-to-maturity yield after ten years	5.33%
Asset backed securities [Member]
Schedule of Available-for-sale 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	$ 226
Available-for-sale yield after ten years	3.32%
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	$ 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	$ 9
Held-to-maturity yield after ten years	6.53%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 406
Available-for-sale yield within one year	2.35%
Available-for-sale amount after one but within five years	$ 3,297
Available-for-sale yield after one but within five years	1.34%
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%