Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 671
Available-for-sale yield within one year	1.68%
Available-for-sale amount after one but within five years	$ 9,826
Available-for-sale yield after one but within five years	1.85%
Available-for-sale amount after five but within ten years	$ 8,201
Available-for-sale yield after five but within ten years	2.29%
Available-for-sale amount after ten years	$ 19,668
Available-for-sale yield after ten years	2.61%
Total fair value within one year	$ 671
Total fair value after one but within five years	10,046
Total fair value after five but within ten years	8,361
Total fair value after ten years	19,537
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year	3.67%
Held-to-maturity amount after one but within five years	$ 128
Held-to-maturity yield after one but within five years	1.53%
Held-to-maturity after five but within ten years	$ 625
Held-to-maturity yield after five but within ten years	2.77%
Held-to-maturity after ten years	$ 12,880
Held-to-maturity yield after ten years	2.55%
Total fair value within one year	$ 3
Total fair value after one but within five years	130
Total fair value after five but within ten years	649
Total fair value after ten years	13,184
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	$ 6,198
Available-for-sale yield after one but within five years	1.30%
Available-for-sale amount after five but within ten years	$ 7,272
Available-for-sale yield after five but within ten years	2.28%
Available-for-sale amount after ten years	$ 4,681
Available-for-sale yield after ten years	2.72%
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	$ 3,407
Available-for-sale yield after one but within five years	2.85%
Available-for-sale amount after five but within ten years	$ 898
Available-for-sale yield after five but within ten years	2.33%
Available-for-sale amount after ten years	$ 2,272
Available-for-sale yield after ten years	2.97%
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	$ 112
Held-to-maturity yield after one but within five years	1.41%
Held-to-maturity after five but within ten years	$ 590
Held-to-maturity yield after five but within ten years	2.70%
Held-to-maturity after ten years	$ 3,833
Held-to-maturity yield after ten years	2.95%
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	$ 5
Available-for-sale yield after one but within five years	3.91%
Available-for-sale amount after five but within ten years	$ 31
Available-for-sale yield after five but within ten years	3.84%
Available-for-sale amount after ten years	$ 12,511
Available-for-sale yield after ten years	2.50%
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	$ 11
Held-to-maturity yield after one but within five years	1.42%
Held-to-maturity after five but within ten years	$ 30
Held-to-maturity yield after five but within ten years	3.92%
Held-to-maturity after ten years	$ 9,035
Held-to-maturity yield after ten years	2.38%
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	$ 25
Available-for-sale yield after one but within five years	4.67%
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%
Held-to-maturity amount within one year	$ 3
Held-to-maturity yield within one year	3.67%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	4.38%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	3.85%
Held-to-maturity after ten years	$ 5
Held-to-maturity yield after ten years	4.47%
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 400
Available-for-sale yield within one year	1.88%
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	$ 204
Available-for-sale yield after ten years	3.21%
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	$ 7
Held-to-maturity yield after ten years	6.04%
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale amount within one year	$ 271
Available-for-sale yield within one year	1.39%
Available-for-sale amount after one but within five years	$ 191
Available-for-sale yield after one but within five years	1.27%
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%