Leases - Summary of Scheduled Future Minimum Lease Payments under Operating and Finance Leases (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	[1]
Operating Leases
Last six months of 2019	$ 428
2020	989
2021	766
2022	634
2023	621
After 2023	4,331
Minimum lease payments	7,769
Imputed interest	2,098
Present value of minimum lease payments	5,671
Less: current maturities of lease obligations	(637)	$ (719)
Long-term lease obligations	5,034	5,276
Finance Leases
Last six months of 2019	157
2020	58
2021	54
2022	44
2023	33
After 2023	70
Minimum lease payments	416
Imputed interest	97
Present value of minimum lease payments	319
Less: current maturities of lease obligations	(117)	(123)
Long-term lease obligations	$ 202	$ 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.