Financial Instruments (Estimated Fair Value of Debt, Financing and Capital Lease Obligations) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	$ 40,193	$ 40,820
Liabilities, Fair Value Disclosure, Nonrecurring (Deprecated 2018-01-31)		41,286
Quoted prices in active markets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring (Deprecated 2018-01-31)		37,549
Observable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring (Deprecated 2018-01-31)		0
Unobservable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Nonrecurring (Deprecated 2018-01-31)		$ 3,737
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	40,809
Fair Value, Measurements, Nonrecurring [Member] | Quoted prices in active markets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	36,642
Fair Value, Measurements, Nonrecurring [Member] | Observable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	197
Fair Value, Measurements, Nonrecurring [Member] | Unobservable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current and long-term debt, carrying amount	$ 3,970