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v3.20.2

MGP, Landfill and Disposal Sites (Details) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
Other MGP, Landfill, or Disposal Sites [Domain] |  |
Manufactured Gas Plant (MGP) Site [Abstract] |  |
Number of identified MGP, landfill, or disposal sites under current investigation and/or remediation | 13 |
NSP-Wisconsin | Ashland MGP Site |  |
Manufactured Gas Plant (MGP) Site [Abstract] |  |
Current Cost Estimate for Site Remediation | $ 199 |
Accrual for Environmental Loss Contingencies, Gross | $ 20 | $ 23
Carrying cost percentage to be applied to unamortized regulatory asset | 3.00% |

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