Securities - Maturity Distribution by Carrying Amount and Yield (on Tax Equivalent Basis) of Investment Securities Portfolio (Detail) $ in Millions	Mar. 31, 2016 USD ($)
Securities available-for-sale:
One year or less	$ 8,024
Over 1 through 5 years	21,044
Over 5 through 10 years	4,645
Over 10 years	4,115
Total	76,294
Securities held-to-maturity:
One year or less	1,844
Over 1 through 5 years	9,896
Over 5 through 10 years	3,322
Over 10 years	15
Total	41,717
Mortgage-backed securities
Securities available-for-sale:
Without single maturity date	32,770
Securities held-to-maturity:
Mortgage-backed securities	26,640
Asset-backed securities
Securities available-for-sale:
Without single maturity date	4,832
Equity securities
Securities available-for-sale:
Without single maturity date	864	[1]
U.S. Treasury
Securities available-for-sale:
One year or less	2,641
Over 1 through 5 years	5,284
Over 5 through 10 years	1,381
Over 10 years	3,666
Total	$ 12,972
Securities available-for-sale (percent):
One year or less (percent)	0.56%	[2]
Over 1 through 5 years (percent)	1.49%	[2]
Over 5 through 10 years (percent)	1.93%	[2]
Over 10 years (percent)	3.11%	[2]
Total (percent)	1.80%	[2]
Securities held-to-maturity:
One year or less	$ 1,601
Over 1 through 5 years	7,165
Over 5 through 10 years	2,561
Over 10 years	0
Total	$ 11,327
Securities held-to-maturity (percent):
One year or less (percent)	0.98%	[2]
Over 1 through 5 years (percent)	1.08%	[2]
Over 5 through 10 years (percent)	2.06%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.29%	[2]
U.S. Government agencies
Securities available-for-sale:
One year or less	$ 50
Over 1 through 5 years	187
Over 5 through 10 years	173
Over 10 years	0
Total	$ 410
Securities available-for-sale (percent):
One year or less (percent)	2.56%	[2]
Over 1 through 5 years (percent)	1.09%	[2]
Over 5 through 10 years (percent)	2.46%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.85%	[2]
Securities held-to-maturity:
One year or less	$ 25
Over 1 through 5 years	1,463
Over 5 through 10 years	0
Over 10 years	0
Total	$ 1,488
Securities held-to-maturity (percent):
One year or less (percent)	0.64%	[2]
Over 1 through 5 years (percent)	1.05%	[2]
Over 5 through 10 years (percent)	0.00%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	1.05%	[2]
State and political subdivisions
Securities available-for-sale:
One year or less	$ 470
Over 1 through 5 years	1,970
Over 5 through 10 years	1,055
Over 10 years	226
Total	$ 3,721
Securities available-for-sale (percent):
One year or less (percent)	2.32%	[2]
Over 1 through 5 years (percent)	2.73%	[2]
Over 5 through 10 years (percent)	4.06%	[2]
Over 10 years (percent)	1.52%	[2]
Total (percent)	2.98%	[2]
Securities held-to-maturity:
One year or less	$ 0
Over 1 through 5 years	1
Over 5 through 10 years	4
Over 10 years	15
Total	$ 20
Securities held-to-maturity (percent):
One year or less (percent)	0.00%	[2]
Over 1 through 5 years (percent)	7.01%	[2]
Over 5 through 10 years (percent)	6.80%	[2]
Over 10 years (percent)	5.33%	[2]
Total (percent)	5.74%	[2]
Other bonds, notes and debentures
Securities available-for-sale:
One year or less	$ 4,863
Over 1 through 5 years	13,603
Over 5 through 10 years	2,036
Over 10 years	223
Total	$ 20,725
Securities available-for-sale (percent):
One year or less (percent)	1.01%	[2]
Over 1 through 5 years (percent)	1.02%	[2]
Over 5 through 10 years (percent)	1.31%	[2]
Over 10 years (percent)	1.69%	[2]
Total (percent)	1.06%	[2]
Securities held-to-maturity:
One year or less	$ 218
Over 1 through 5 years	1,267
Over 5 through 10 years	757
Over 10 years	0
Total	$ 2,242
Securities held-to-maturity (percent):
One year or less (percent)	0.43%	[2]
Over 1 through 5 years (percent)	0.63%	[2]
Over 5 through 10 years (percent)	0.70%	[2]
Over 10 years (percent)	0.00%	[2]
Total (percent)	0.64%	[2]
Mortgage/ asset-backed and equity securities
Securities available-for-sale:
Total	$ 38,466
Securities available-for-sale (percent):
Total (percent)	2.81%	[2]
Securities held-to-maturity:
Total	$ 26,640
Securities held-to-maturity (percent):
Total (percent)	2.72%	[2]
Mortgage/ asset-backed and equity securities | Mortgage-backed securities
Securities available-for-sale:
Without single maturity date	$ 32,770
Securities available-for-sale (percent):
Without single maturity date (percent)	3.07%	[2]
Securities held-to-maturity:
Mortgage-backed securities	$ 26,640
Securities held-to-maturity (percent):
Without single maturity date (percent)	2.72%	[2]
Mortgage/ asset-backed and equity securities | Asset-backed securities
Securities available-for-sale:
Without single maturity date	$ 4,832
Securities available-for-sale (percent):
Without single maturity date (percent)	1.55%	[2]
Mortgage/ asset-backed and equity securities | Equity securities
Securities available-for-sale:
Without single maturity date	$ 864	[1]
Securities available-for-sale (percent):
Without single maturity date (percent)	0.00%	[1],[2]

[1] Includes money market funds.

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