Leases - Future Lease Payments Under Non-cancelable Leases (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
ASC 842 Operating Leases
2019 (excluding the nine months ended September 30, 2019)	$ 43
2020	191
2021	139
2022	99
2023	69
Thereafter	150
Total future undiscounted lease payments	691
Less imputed interest	(89)
Total operating lease liabilities	602
ASC 842 Finance Leases
2019 (excluding the nine months ended September 30, 2019)	7
2020	29
2021	24
2022	12
2023	7
Thereafter	11
Total future undiscounted lease payments	90
Less imputed interest	(8)
Total finance lease liabilities	$ 82
ASC 840 Operating Leases
2019		$ 208
2020		165
2021		114
2022		79
2023		57
Thereafter		157
Total future undiscounted lease payments		$ 780