Leases - Summary of Scheduled Future Minimum Lease Payments under Operating and Finance Leases (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	[1]
Operating Leases
Last nine months of 2019	$ 662
2020	985
2021	763
2022	630
2023	617
After 2023	4,252
Minimum lease payments	7,909
Imputed interest	2,125
Present value of minimum lease payments	5,784
Less: current maturities of lease obligations	(639)	$ (719)
Long-term lease obligations	5,145	5,276
Finance Leases
Last nine months of 2019	195
2020	96
2021	74
2022	47
2023	33
After 2023	69
Minimum lease payments	514
Imputed interest	145
Present value of minimum lease payments	369
Less: current maturities of lease obligations	(133)	(123)
Long-term lease obligations	$ 236	$ 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.