Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020
Assets:
ROU operating lease assets	$ 1,459	$ 1,428
ROU finance lease assets	971	1,220
Total	2,430	2,648
Current
Operating lease	450	482
Finance lease	431	444
Total	881	926
Non-current
Operating lease	1,113	1,063
Finance lease	545	602
Total	$ 1,658	$ 1,665
Weighted Average remaining lease term:
Operating leases	4 years 9 months 18 days	4 years 9 months 18 days
Finance leases	2 years 8 months 12 days	2 years 8 months 12 days
Weighted average remaining discount rate:
Operating leases	4.00%	4.00%
Finance leases	3.80%	6.40%
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:DebtCurrent	us-gaap:DebtCurrent
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtNoncurrent	us-gaap:LongTermDebtNoncurrent