Leases - Undiscounted Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Operating Leases
2020	$ 1,003
2021	836
2022	729
2023	698
2024	628
Thereafter	3,821
Total minimum lease payments	7,715
Less: amount of lease payments representing interest	(1,620)
Present value of future minimum lease payments	6,095
Less: current obligations under leases	(801)	$ (955)
Noncurrent operating lease liabilities	5,294	5,801
Finance Leases
2020	264
2021	239
2022	174
2023	124
2024	180
Thereafter	179
Total minimum lease payments	1,160
Less: amount of lease payments representing interest	(106)
Present value of future minimum lease payments	1,054
Less: current obligations under leases	(233)	(109)
Long-term lease obligations	$ 821	$ 294