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Commitments and Contingencies Environmental Requirements - Water and Waste (Details) - Federal Coal Ash Regulation [Domain] $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2019
Site Contingency [Line Items] |  |
Number of sites where regulated ash units will still be in operation at a specified date | 9 |
NSP Minnesota [Member] |  |
Site Contingency [Line Items] |  |
Number of sites where statistically significant increases over established groundwater standards exist |  | 0
Number of impoundments where closure plans will be expedited | 1 | 1
Estimated cost of closure of an impoundment | $ 4 |
Estimated cost of construction of a new impoundment | $ 9 |
Time period of existing ash pond closure completion in accordance with the CCR rule | 5 years |
PSCo [Member] |  |
Site Contingency [Line Items] |  |
Number of sites where statistically significant increases over established groundwater standards exist |  | 4
Number of sites where corrective action options are being evaluated for locations with statistically significant increases above background concentrations | 2 |
Number of sites where monitoring results indicate potential offsite impacts to groundwater | 1 |

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