Leases - Future Minimum Lease Payments Due Under ASC 842 Guidance (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Operating Leases
2019 (excluding the six months ended June 30, 2019)	$ 155
2020	259
2021	214
2022	169
2023	126
Thereafter	762
Total future lease payments	1,685
Less imputed interest	333
Present value of future lease payments	1,352
Less current portion of lease liabilities	240	$ 0
Long-term lease liabilities	1,112	$ 0
ROU assets	1,280
Finance Leases
2019 (excluding the six months ended June 30, 2019)	44
2020	81
2021	75
2022	74
2023	28
Thereafter	66
Total future lease payments	368
Less imputed interest	72
Present value of future lease payments	296
Less current portion of lease liabilities	64
Long-term lease liabilities	232
ROU assets	$ 338