Lease Commitments - Assets and Liabilities of Lessee (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Operating lease assets	$ 911	$ 929
Finance lease assets	$ 89	$ 100
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total lease assets	$ 1,000	$ 1,029
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Current lease liabilities
Operating	$ 204	$ 186
Finance	$ 30	$ 24
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Non-current lease liabilities
Operating	$ 897	$ 944
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Finance	$ 65	$ 76
Total lease liabilities	$ 1,196	$ 1,230