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, 2015	Mar. 31, 2014
Interest Rates
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative assets	$ 19	$ 16
Derivative liabilities	2	3
Equity Contract
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative assets	485	348
Derivative liabilities	$ 60	$ 43