Financial Instruments (Estimated Fair Value of Debt, Financing and Capital Lease Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018
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	$ 40,125	$ 40,820
Liabilities, Fair Value Disclosure, Nonrecurring	40,326	41,286
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	35,756	37,549
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring	$ 4,570	$ 3,737