Leases - Summary of Scheduled Future Minimum Lease Payments under Operating and Finance Leases (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	[1]
Operating Leases
Last three months of 2019	$ 195
2020	1,004
2021	788
2022	660
2023	647
After 2023	4,611
Minimum lease payments	7,905
Imputed interest	(2,186)
Present value of minimum lease payments	5,719
Less: current maturities of lease obligations	(778)	$ (719)
Long-term lease obligations	4,941	5,276
Finance Leases
Last three months of 2019	80
2020	58
2021	54
2022	44
2023	33
After 2023	70
Minimum lease payments	339
Imputed interest	(61)
Present value of minimum lease payments	278
Less: current maturities of lease obligations	(92)	(123)
Long-term lease obligations	$ 186	$ 224

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.