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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Ashland MGP Site | NSP-Wisconsin |  |
Manufactured Gas Plant (MGP) Site [Abstract] |  |
Current Cost Estimate for Site Remediation | $ 192 |
Accrual for Environmental Loss Contingencies, Gross | 27 | $ 30
Estimated Amount Spent on Cleanup | $ 165 |
Approved Amortization Period for Recovery of Remediation Costs in Natural Gas Rates | 10 years |
Carrying Cost Percentage to be Applied to Unamortized Regulatory Asset | 3.00% |
Other MGP, Landfill, or Disposal Sites |  |
Manufactured Gas Plant (MGP) Site [Abstract] |  |
Number of Identified MGP, Landfill, or Disposal Sites Under Current Investigation and/or Remediation | 12 |
Accrual for Environmental Loss Contingencies | $ 19 | $ 9
Federal Coal Ash Regulation [Domain] |  |
Site Contingency [Line Items] |  |
Number of sites where regulated ash units will still be in operation at a specified date | 9 |
Federal Coal Ash Regulation [Domain] | PSCo |  |
Manufactured Gas Plant (MGP) Site [Abstract] |  |
Number of sites where statistically significant increases over established groundwater standards exist | 2 |
Federal Coal Ash Regulation [Domain] | NSP-Minnesota |  |
Site Contingency [Line Items] |  |
Estimated cost of construction of a new impoundment | $ 6 |
Number of impoundments where closure plans will be expedited | 1 |

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