Leases - Supplemental balance sheet information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating leases:
Operating lease right of use assets	$ 864	$ 858
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Operating lease right of use assets
Current operating lease liabilities	$ 256	247
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Current operating lease liabilities
Noncurrent operating lease liabilities	$ 609	607
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Noncurrent operating lease liabilities
Present value of future minimum lease payments	$ 865	854
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent us-gaap:OperatingLeaseLiabilityNoncurrent
Finance leases:
Property and equipment, at cost	$ 26,650	26,124
Accumulated amortization	(17,229)	(16,791)
Property, Plant and Equipment - net	9,421	9,333
Current obligations of finance leases	$ 22	21
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Current obligations of finance leases
Finance lease obligations	$ 93	111
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Finance lease obligations
Finance Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent
Present value of future minimum lease payments	$ 115	$ 132
Weighted average remaining lease term (in years):
Operating leases weighted average remaining lease term (in years)	5 years 7 months 6 days	5 years 8 months 12 days
Finance leases weighted average remaining lease term (in years)	7 years 7 months 6 days	9 years
Weighted average discount rate:
Operating leases weighted average discount rate (as a percent)	2.40%	3.20%
Finance leases weighted average discount rate (as a percent)	3.50%	3.80%
Property and equipment finance leases
Finance leases:
Property and equipment, at cost	$ 225	$ 239
Accumulated amortization	(106)	(102)
Property, Plant and Equipment - net	$ 119	$ 137