Securities-Maturity Distribution by Carrying Amount and Yield (on Tax Equivalent Basis) of Investment Securities Portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Securities available-for-sale:
One year or less	$ 9,555
Over 1 through 5 years	25,894
Over 5 through 10 years	6,337
Over 10 years	4,188
Total	87,717
Securities held-to-maturity:
One year or less	1,611
Over 1 through 5 years	10,052
Over 5 through 10 years	3,172
Over 10 years	15
Held-to-maturity (fair value of $41,676 and $21,127)	41,237
Mortgage-backed securities
Securities available-for-sale:
Without single maturity date	35,263
Securities held-to-maturity:
Mortgage-backed securities	26,387
Asset-backed securities
Securities available-for-sale:
Without single maturity date	5,658
Equity securities
Securities available-for-sale:
Without single maturity date	822	[1]
U.S. Treasury
Securities available-for-sale:
One year or less	1,759
Over 1 through 5 years	10,914
Over 5 through 10 years	2,223
Over 10 years	3,690
Total	18,586
Securities held-to-maturity:
One year or less	150
Over 1 through 5 years	7,760
Over 5 through 10 years	2,462
Over 10 years	0
Held-to-maturity (fair value of $41,676 and $21,127)	10,372
Securities available-for-sale (percent):
One year or less (percent)	0.95%	[2]
Over 1 through 5 years (percent)	0.89%	[2]
Over 5 through 10 years (percent)	2.52%	[2]
Over 10 years (percent)	3.11%	[2]
Total (percent)	1.53%	[2]
Securities held-to-maturity (percent):
One year or less (percent)	0.28%	[2]
Over 1 through 5 years (percent)	1.07%	[2]
Over 5 through 10 years (percent)	2.06%	[2]
Over 10 years (percent)	0.00%
Total (percent)	1.30%	[2]
U.S. Government agencies
Securities available-for-sale:
One year or less	126
Over 1 through 5 years	259
Over 5 through 10 years	0
Over 10 years	0
Total	385
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	968
Over 5 through 10 years	200
Over 10 years	0
Held-to-maturity (fair value of $41,676 and $21,127)	1,168
Securities available-for-sale (percent):
One year or less (percent)	1.86%	[2]
Over 1 through 5 years (percent)	1.66%	[2]
Over 5 through 10 years (percent)	0.00%
Over 10 years (percent)	0.00%
Total (percent)	1.73%	[2]
Securities held-to-maturity (percent):
One year or less (percent)	0.00%
Over 1 through 5 years (percent)	1.05%	[2]
Over 5 through 10 years (percent)	1.19%	[2]
Over 10 years (percent)	0.00%
Total (percent)	1.07%	[2]
State and political subdivisions
Securities available-for-sale:
One year or less	462
Over 1 through 5 years	2,938
Over 5 through 10 years	1,506
Over 10 years	233
Total	5,139
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	1
Over 5 through 10 years	5
Over 10 years	15
Held-to-maturity (fair value of $41,676 and $21,127)	21
Securities available-for-sale (percent):
One year or less (percent)	1.40%	[2]
Over 1 through 5 years (percent)	2.26%	[2]
Over 5 through 10 years (percent)	3.80%	[2]
Over 10 years (percent)	1.81%	[2]
Total (percent)	2.62%	[2]
Securities held-to-maturity (percent):
One year or less (percent)	0.00%
Over 1 through 5 years (percent)	7.32%	[2]
Over 5 through 10 years (percent)	6.95%	[2]
Over 10 years (percent)	3.77%	[2]
Total (percent)	4.74%	[2]
Other bonds, notes and debentures
Securities available-for-sale:
One year or less	7,208
Over 1 through 5 years	11,783
Over 5 through 10 years	2,608
Over 10 years	265
Total	21,864
Securities held-to-maturity:
One year or less	1,461
Over 1 through 5 years	1,323
Over 5 through 10 years	505
Over 10 years	0
Held-to-maturity (fair value of $41,676 and $21,127)	3,289
Securities available-for-sale (percent):
One year or less (percent)	0.58%	[2]
Over 1 through 5 years (percent)	1.03%	[2]
Over 5 through 10 years (percent)	2.19%	[2]
Over 10 years (percent)	1.45%	[2]
Total (percent)	1.02%	[2]
Securities held-to-maturity (percent):
One year or less (percent)	0.24%	[2]
Over 1 through 5 years (percent)	0.58%	[2]
Over 5 through 10 years (percent)	0.88%	[2]
Over 10 years (percent)	0.00%
Total (percent)	0.48%	[2]
Mortgage/ asset-backed and equity securities
Securities available-for-sale:
Total	41,743
Securities held-to-maturity:
Held-to-maturity (fair value of $41,676 and $21,127)	26,387
Securities available-for-sale (percent):
Total (percent)	2.42%	[2]
Securities held-to-maturity (percent):
Total (percent)	2.65%	[2]
Mortgage/ asset-backed and equity securities | Mortgage-backed securities
Securities available-for-sale:
Without single maturity date	35,263
Securities held-to-maturity:
Mortgage-backed securities	26,387
Securities available-for-sale (percent):
Without single maturity date (percent)	2.69%	[2]
Securities held-to-maturity (percent):
Without single maturity date (percent)	2.65%	[2]
Mortgage/ asset-backed and equity securities | Asset-backed securities
Securities available-for-sale:
Without single maturity date	5,658
Securities available-for-sale (percent):
Without single maturity date (percent)	1.07%	[2]
Mortgage/ asset-backed and equity securities | Equity securities
Securities available-for-sale:
Without single maturity date	$ 822	[1]
Securities available-for-sale (percent):
Without single maturity date (percent)	0.00%

[1] Includes money market funds.

[2] Yields are based upon the amortized cost of securities.