Leases - Components of Lease Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Operating lease cost	$ 616	$ 698
Short-term lease cost	53	49
Variable lease cost	56	46
Sublease income	(40)	(45)
Total operating costs	685	748
Finance lease cost:
Amortization of right-of-use assets	433	405
Interest on lease liabilities	45	65
Total finance lease cost	$ 478	$ 470