Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Income Statement) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Total interest expense	$ 1,300	$ 1,614	$ 2,023