Financial Instruments (Estimated Fair Value of Debt, Financing and Capital Lease Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, financing and capital lease obligations, Carrying amount	$ 36,690	$ 40,581
Liabilities, Fair Value Disclosure, Nonrecurring	38,205	43,016
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	30,281	33,196
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	2,970	4,352
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	$ 4,954	$ 5,468