Leases - Supplemental Balance Sheet Information (Details) $ in Millions	Sep. 30, 2020 USD ($)
Leases [Abstract]
Right-of-use asset	$ 137
Current portion of long-term lease liability	79
Long-term lease liability	$ 61
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	mro:Deferredcreditandotherliabilities