Financial Instruments (Estimated Fair Value of Debt, Financing and Capital Lease Obligations) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	$ 33,434	$ 32,277
Current and long-term debt, total estimated fair value	33,554	34,199
Quoted prices in active markets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, total estimated fair value	27,238	27,516
Observable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, total estimated fair value	4,906	5,421
Unobservable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, total estimated fair value	$ 1,410	$ 1,262