Credit Loss Reserves - Summarizes Changes in Credit Loss Reserves by Product/Class and Related Receivable Balance by Product (Parenthetical) (Detail) (USD $)	Mar. 31, 2013	Mar. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Excludes Credit loss reserves for TDR Loans that are carried at the lower of amortized cost or fair value	$ 107,000,000	$ 204,000,000
TDR [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Excludes TDR Loans that are carried at the lower of amortized cost or fair value	$ 1,700,000,000	$ 3,100,000,000