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, 2013
Securities available-for-sale:
One year or less	$ 8,144
Over 1 through 5 years	22,512
Over 5 through 10 years	7,282
Over 10 years	4,525
Total	94,878
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	1,307
Over 5 through 10 years	1,064
Over 10 years	15
Total	11,678
Mortgage-Backed Securities
Securities available-for-sale:
Without single maturity date	46,448
Securities held-to-maturity:
Without single maturity date	9,292
Asset-Backed Securities
Securities available-for-sale:
Without single maturity date	3,482
Equity securities
Securities available-for-sale:
Without single maturity date	2,485	[1]
U.S. Treasury
Securities available-for-sale:
One year or less	4,332
Over 1 through 5 years	7,204
Over 5 through 10 years	2,041
Over 10 years	4,215
Total	17,792
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	1,180
Over 5 through 10 years	1,026
Over 10 years	0
Total	2,206
Securities available-for-sale:
One year or less (percent)	0.66%	[2]
Over 1 through 5 years (percent)	1.03%	[2]
Over 5 through 10 years (percent)	2.70%	[2]
Over 10 years (percent)	3.12%	[2]
Total (percent)	1.63%	[2]
Securities held-to-maturity:
One year or less (percent)	0.00%	[2]
Over 1 through 5 years (percent)	1.24%	[2]
Over 5 through 10 years (percent)	2.25%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.71%	[2]
U.S. Government agencies
Securities available-for-sale:
One year or less	328
Over 1 through 5 years	677
Over 5 through 10 years	55
Over 10 years	0
Total	1,060
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	0
Over 5 through 10 years	0
Over 10 years	0
Total	0
Securities available-for-sale:
One year or less (percent)	1.14%	[2]
Over 1 through 5 years (percent)	1.85%	[2]
Over 5 through 10 years (percent)	2.06%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.64%	[2]
Securities held-to-maturity:
One year or less (percent)	0.00%	[2]
Over 1 through 5 years (percent)	0.00%	[2]
Over 5 through 10 years (percent)	0.00%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	0.00%	[2]
State and political subdivisions
Securities available-for-sale:
One year or less	151
Over 1 through 5 years	3,064
Over 5 through 10 years	2,737
Over 10 years	298
Total	6,250
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	0
Over 5 through 10 years	38
Over 10 years	15
Total	53
Securities available-for-sale:
One year or less (percent)	1.18%	[2]
Over 1 through 5 years (percent)	1.79%	[2]
Over 5 through 10 years (percent)	3.24%	[2]
Over 10 years (percent)	2.72%	[2]
Total (percent)	2.46%	[2]
Securities held-to-maturity:
One year or less (percent)	0.00%	[2]
Over 1 through 5 years (percent)	0.00%	[2]
Over 5 through 10 years (percent)	6.78%	[2]
Over 10 years (percent)	5.81%	[2]
Total (percent)	6.51%	[2]
Other bonds, Notes and Debentures
Securities available-for-sale:
One year or less	3,333
Over 1 through 5 years	11,567
Over 5 through 10 years	2,449
Over 10 years	12
Total	17,361
Securities held-to-maturity:
One year or less	0
Over 1 through 5 years	127
Over 5 through 10 years	0
Over 10 years	0
Total	127
Securities available-for-sale:
One year or less (percent)	1.11%	[2]
Over 1 through 5 years (percent)	1.04%	[2]
Over 5 through 10 years (percent)	2.54%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.27%	[2]
Securities held-to-maturity:
One year or less (percent)	0.00%	[2]
Over 1 through 5 years (percent)	0.53%	[2]
Over 5 through 10 years (percent)	0.00%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	0.53%	[2]
Mortgage/Asset-Backed and Equity Securities
Securities available-for-sale:
Total	52,415
Securities held-to-maturity:
Total	9,292
Securities available-for-sale:
Total (percent)	2.43%	[2]
Securities held-to-maturity:
Total (percent)	2.76%	[2]
Mortgage/Asset-Backed and Equity Securities | Mortgage-Backed Securities
Securities available-for-sale:
Without single maturity date	46,448
Securities held-to-maturity:
Without single maturity date	9,292
Securities available-for-sale:
Without single maturity date (percent)	2.65%	[2]
Securities held-to-maturity:
Without single maturity date (percent)	2.76%	[2]
Mortgage/Asset-Backed and Equity Securities | Asset-Backed Securities
Securities available-for-sale:
Without single maturity date	3,482
Securities available-for-sale:
Without single maturity date (percent)	1.28%	[2]
Mortgage/Asset-Backed and Equity Securities | Equity securities
Securities available-for-sale:
Without single maturity date	$ 2,485	[1]
Securities available-for-sale:
Without single maturity date (percent)	0.00%	[1],[2]

[1] Includes money market funds.

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