Leases - Supplemental balance sheet information (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Operating leases:
Operating lease right of use assets	$ 834
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Operating lease right of use assets
Current operating lease liabilities	$ 241
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Current operating lease liabilities
Noncurrent operating lease liabilities	$ 584
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Noncurrent operating lease liabilities
Present value of future minimum lease payments	$ 825
Finance leases:
Property and equipment, at cost	25,508	$ 24,873
Accumulated amortization	(16,617)	(16,135)
Property, Plant and Equipment - net	8,891	$ 8,738
Current obligations of finance leases	$ 18
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Current obligations of finance leases
Finance leases, net of current obligations	$ 116
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Finance leases, net of current obligations
Present value of future minimum lease payments	$ 134
Weighted average remaining lease term (in years):
Operating leases weighted average remaining lease term (in years)	5 years 8 months 12 days
Finance leases weighted average remaining lease term (in years)	9 years 2 months 12 days
Weighted average discount rate:
Operating leases weighted average discount rate (as a percent)	3.30%
Finance leases weighted average discount rate (as a percent)	3.80%
Property and equipment finance leases
Finance leases:
Property and equipment, at cost	$ 235
Accumulated amortization	(99)
Property, Plant and Equipment - net	$ 136