Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 1,026
Available-for-sale yield within one year	1.57%
Available-for-sale amount after one but within five years	$ 6,933
Available-for-sale yield after one but within five years	2.42%
Available-for-sale amount after five but within ten years	$ 10,271
Available-for-sale yield after five but within ten years	2.18%
Available-for-sale amount after ten years	$ 12,529
Available-for-sale yield after ten years	2.80%
Total fair value within one year	$ 1,030
Total fair value after one but within five years	6,965
Total fair value after five but within ten years	9,938
Total fair value after ten years	12,290
Held-to-maturity amount within one year	$ 1
Held-to-maturity yield within one year	4.25%
Held-to-maturity amount after one but within five years	$ 347
Held-to-maturity yield after one but within five years	2.66%
Held-to-maturity after five but within ten years	$ 316
Held-to-maturity yield after five but within ten years	2.87%
Held-to-maturity after ten years	$ 13,610
Held-to-maturity yield after ten years	2.55%
Total fair value within one year	$ 1
Total fair value after one but within five years	344
Total fair value after five but within ten years	313
Total fair value after ten years	13,347
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	$ 3,392
Available-for-sale yield after one but within five years	2.22%
Available-for-sale amount after five but within ten years	$ 8,255
Available-for-sale yield after five but within ten years	2.07%
Available-for-sale amount after ten years	$ 3,200
Available-for-sale yield after ten years	3.04%
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 225
Available-for-sale yield within one year	3.54%
Available-for-sale amount after one but within five years	$ 2,588
Available-for-sale yield after one but within five years	3.03%
Available-for-sale amount after five but within ten years	$ 1,998
Available-for-sale yield after five but within ten years	2.64%
Available-for-sale amount after ten years	$ 2,877
Available-for-sale yield after ten years	2.79%
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	$ 322
Held-to-maturity yield after one but within five years	2.57%
Held-to-maturity after five but within ten years	$ 280
Held-to-maturity yield after five but within ten years	2.77%
Held-to-maturity after ten years	$ 2,627
Held-to-maturity yield after ten years	2.86%
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	$ 50
Available-for-sale yield within one year	1.87%
Available-for-sale amount after one but within five years	$ 67
Available-for-sale yield after one but within five years	2.06%
Available-for-sale amount after five but within ten years	$ 18
Available-for-sale yield after five but within ten years	4.00%
Available-for-sale amount after ten years	$ 6,297
Available-for-sale yield after ten years	2.64%
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	$ 20
Held-to-maturity yield after one but within five years	3.86%
Held-to-maturity after five but within ten years	$ 31
Held-to-maturity yield after five but within ten years	3.57%
Held-to-maturity after ten years	$ 10,979
Held-to-maturity yield after ten years	2.47%
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	$ 1
Held-to-maturity yield within one year	4.25%
Held-to-maturity amount after one but within five years	$ 5
Held-to-maturity yield after one but within five years	3.61%
Held-to-maturity after five but within ten years	$ 5
Held-to-maturity yield after five but within ten years	4.57%
Held-to-maturity after ten years	$ 1
Held-to-maturity yield after ten years	4.41%
Asset backed securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 200
Available-for-sale yield within one year	3.17%
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	$ 155
Available-for-sale yield after ten years	4.22%
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	$ 3
Held-to-maturity yield after ten years	6.25%
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Available-for-sale amount within one year	$ 551
Available-for-sale yield within one year	0.16%
Available-for-sale amount after one but within five years	$ 886
Available-for-sale yield after one but within five years	1.40%
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%