Leases - Components of Lease Expense (Details) $ in Millions	12 Months Ended
	Mar. 31, 2020 USD ($)
Leases [Abstract]
Operating lease cost	$ 698
Short-term lease cost	49
Variable lease cost	46
Sublease income	(45)
Total operating costs	748
Finance lease cost:
Amortization of right-of-use assets	405
Interest on lease liabilities	65
Total finance lease cost	$ 470