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Leases (Future Minimum Operating Lease Payments, Excluding Contingent Rentals) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Operating Leased Assets [Line Items]  
2018 $ 6
2019 5
2020 2
2021 2
2022 1
Thereafter 13
Total future minimum operating lease payments, excluding contingent rentals 29
IPL [Member]  
Operating Leased Assets [Line Items]  
2018 3
2019 2
2020 1
2021 1
2022 1
Thereafter 13
Total future minimum operating lease payments, excluding contingent rentals 21
WPL [Member]  
Operating Leased Assets [Line Items]  
2018 3
2019 3
2020 1
2021 0
2022 0
Thereafter 0
Total future minimum operating lease payments, excluding contingent rentals $ 7