Leases - Reconciliation of Undiscounted Cash Flows to the Operating and Finance Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases
2021	$ 298
2022	252
2023	192
2024	156
2025	107
Thereafter	705
Total future lease payments	1,710
Less imputed interest	299
Present value of future lease payments	1,411
Less current portion of lease liabilities	262	$ 252
Long-term lease liabilities	1,149	1,251
ROU assets	1,328	1,432
Finance Leases
2021	87
2022	86
2023	38
2024	21
2025	19
Thereafter	145
Total future lease payments	396
Less imputed interest	73
Present value of future lease payments	323
Less current portion of lease liabilities	68	67
Long-term lease liabilities	255	287
ROU assets	$ 333	$ 391