Financial Instruments (Estimated Fair Value of Debt, Financing and Capital Lease Obligations) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Mar. 31, 2019
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	$ 37,730	$ 40,193
Current and long-term debt and financing obligations, Estimated fair value	40,159	40,809
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt and financing obligations, Estimated fair value	36,253	36,642
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt and financing obligations, Estimated fair value	197	197
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt and financing obligations, Estimated fair value	$ 3,709	$ 3,970