Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Assets:
ROU operating lease assets	$ 1,366	$ 1,428
ROU finance lease assets, balance sheet location	Property and equipment, net of accumulated depreciation of $4,121 and $3,818	Property and equipment, net of accumulated depreciation of $4,121 and $3,818
ROU finance lease assets	$ 834	$ 1,220
Total	2,200	2,648
Current
Operating lease	$ 418	$ 482
Finance lease, balance sheet location	Short-term debt and current maturities of long-term debt	Short-term debt and current maturities of long-term debt
Finance lease	$ 398	$ 444
Total	816	926
Non-current
Operating lease	$ 1,038	$ 1,063
Finance lease, balance sheet location	us-gaap:LongTermDebtNoncurrent	us-gaap:LongTermDebtNoncurrent
Finance lease	$ 496	$ 602
Total	$ 1,534	$ 1,665
Weighted average remaining lease term:
Operating leases	4 years 10 months 24 days	4 years 9 months 18 days
Finance leases	2 years 7 months 6 days	2 years 8 months 12 days
Weighted average remaining discount rate:
Operating leases	3.80%	4.00%
Finance leases	3.60%	6.40%