Loans and Allowance for Credit Losses - Loans Modified as TDRs (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 SecurityLoan	Mar. 31, 2012 SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	12,674	18,660
Pre-Modification Outstanding Loan Balance	$ 551	$ 820
Post-Modification Outstanding Loan Balance	536	795
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	815	1,279
Pre-Modification Outstanding Loan Balance	34	91
Post-Modification Outstanding Loan Balance	33	72
Commercial real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	63	111
Pre-Modification Outstanding Loan Balance	80	204
Post-Modification Outstanding Loan Balance	78	197
Residential mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	807	621
Pre-Modification Outstanding Loan Balance	110	111
Post-Modification Outstanding Loan Balance	105	107
Credit card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	7,818	14,218
Pre-Modification Outstanding Loan Balance	48	80
Post-Modification Outstanding Loan Balance	48	80
Other retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	1,865	988
Pre-Modification Outstanding Loan Balance	49	15
Post-Modification Outstanding Loan Balance	49	15
Total loans, excluding GNMA and covered loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	11,368	17,217
Pre-Modification Outstanding Loan Balance	321	501
Post-Modification Outstanding Loan Balance	313	471
Loans purchased from GNMA mortgage pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	1,256	1,400
Pre-Modification Outstanding Loan Balance	177	179
Post-Modification Outstanding Loan Balance	182	187
Covered loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	50	43
Pre-Modification Outstanding Loan Balance	53	140
Post-Modification Outstanding Loan Balance	$ 41	$ 137