Leases - Future Maturities of Lease Liabilities - ASC 842 (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Operating Leases
2019 (excluding the three months ended March 31)	$ 190
2020	213
2021	162
2022	113
2023	86
Thereafter	359
Total Lease Payments	1,123
Less: Imputed Interest	253
Total	870
Finance Leases
2019 (excluding the three months ended March 31)	19
2020	23
2021	34
2022	21
2023	20
Thereafter	706
Total Lease Payments	823
Less: Imputed Interest	578
Total	$ 245	$ 37