Leases - Undiscounted Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases
2021	$ 949
2022	848
2023	837
2024	770
2025	746
Thereafter	4,450
Total minimum lease payments	8,600
Less: amount of lease payments representing interest	(2,209)
Present value of future minimum lease payments	6,391
Less: current obligations under leases	(678)	$ (801)
Noncurrent operating lease liabilities	5,713	5,294
Finance Leases
2021	328
2022	238
2023	191
2024	266
2025	128
Thereafter	149
Total minimum lease payments	1,300
Less: amount of lease payments representing interest	(117)
Present value of future minimum lease payments	1,183
Less: current obligations under leases	(289)	(233)
Long-term lease obligations	$ 894	$ 821