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Leases - Right-of-Way and Operating Lease Maturities Under Topic 840 (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Lessee, Lease, Description [Line Items]  
2019 $ 832
2020 577
2021 467
2022 399
2023 356
2024 1,724
Total future minimum payments 4,355
Future minimum sublease rentals 101
Right-of-Way Agreements  
Lessee, Lease, Description [Line Items]  
2019 157
2020 134
2021 112
2022 120
2023 115
2024 755
Total future minimum payments 1,393
Operating Leases  
Lessee, Lease, Description [Line Items]  
2019 675
2020 443
2021 355
2022 279
2023 241
2024 969
Total future minimum payments $ 2,962