Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Interest Rates
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivative assets	$ 51	$ 21
Derivative liabilities	3	4
Equity Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivative assets	211	159
Derivative liabilities	$ 37	$ 24