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Leases - Future Minimum Lease Commitments Before Adoption of ASU 2016-02 (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Operating Leased Assets [Line Items]  
Future sublease income $ 29
Powerton and Joliet  
Operating Leased Assets [Line Items]  
2019 1
2020 1
2021 3
2022 6
2023 6
Thereafter 222
Total (a) 239
Other Leased Property  
Operating Leased Assets [Line Items]  
2019 60
2020 55
2021 43
2022 40
2023 39
Thereafter 95
Total (a) $ 332