Debt and Financing Activities - Commercial Paper (Details) - USD ($)	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Debt Instrument [Line Items]
Commercial paper issuances	$ 21,400,000,000	$ 37,300,000,000
Commercial paper repaid	21,400,000,000	37,300,000,000
Total debt outstanding	7,387,000,000	7,595,000,000
Commercial Paper
Debt Instrument [Line Items]
Credit facility borrowing capacity (up to)	4,000,000,000.0
Total debt outstanding	$ 0	$ 0