Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 567
Available-for-sale yield within one year	3.16%
Available-for-sale amount after one but within five years	9,840
Available-for-sale yield after one but within five years	1.50%
Available-for-sale amount after five but within ten years	5,288
Available-for-sale yield after five but within ten years	2.46%
Available-for-sale amount after ten years	16,806
Available-for-sale yield after ten years	2.88%
Total fair value within one year	574
Total fair value after one but within five years	9,954
Total fair value after five but within ten years	5,479
Total fair value after ten years	16,924
Held-to-maturity amount within one year	3
Held-to-maturity yield within one year	5.09%
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	668
Held-to-maturity yield after five but within ten years	2.55%
Held-to-maturity after ten years	13,243
Held-to-maturity yield after ten years	2.64%
Total fair value within one year	3
Total fair value after one but within five years	103
Total fair value after five but within ten years	685
Total fair value after ten years	13,559
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	4,124
Available-for-sale yield after one but within five years	0.70%
Available-for-sale amount after five but within ten years	3,227
Available-for-sale yield after five but within ten years	2.24%
Available-for-sale amount after ten years	3,369
Available-for-sale yield after ten years	2.81%
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	2,057
Available-for-sale yield after one but within five years	3.08%
Available-for-sale amount after five but within ten years	1,723
Available-for-sale yield after five but within ten years	2.77%
Available-for-sale amount after ten years	1,068
Available-for-sale yield after ten years	3.27%
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	1.40%
Held-to-maturity after five but within ten years	615
Held-to-maturity yield after five but within ten years	2.52%
Held-to-maturity after ten years	4,573
Held-to-maturity yield after ten years	3.01%
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	9
Available-for-sale yield after one but within five years	4.22%
Available-for-sale amount after five but within ten years	42
Available-for-sale yield after five but within ten years	3.89%
Available-for-sale amount after ten years	11,784
Available-for-sale yield after ten years	2.84%
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	7.66%
Held-to-maturity after five but within ten years	47
Held-to-maturity yield after five but within ten years	2.80%
Held-to-maturity after ten years	8,656
Held-to-maturity yield after ten years	2.44%
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	96
Available-for-sale yield after one but within five years	4.12%
Available-for-sale amount after five but within ten years	296
Available-for-sale yield after five but within ten years	2.96%
Available-for-sale amount after ten years	350
Available-for-sale yield after ten years	3.41%
Held-to-maturity amount within one year	3
Held-to-maturity yield within one year	5.09%
Held-to-maturity amount after one but within five years	8
Held-to-maturity yield after one but within five years	3.93%
Held-to-maturity after five but within ten years	6
Held-to-maturity yield after five but within ten years	3.83%
Held-to-maturity after ten years	4
Held-to-maturity yield after ten years	5.34%
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	235
Available-for-sale yield after ten years	3.20%
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	10
Held-to-maturity yield after ten years	6.40%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	567
Available-for-sale yield within one year	3.16%
Available-for-sale amount after one but within five years	3,554
Available-for-sale yield after one but within five years	1.42%
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%