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Commitments and Contingencies NSP-Minnesota-Sherco (Details) - USD ($) $ in Millions | 1 Months Ended | 6 Months Ended
--- | --- | ---
Jan. 31, 2019 | Jun. 30, 2019
General Electric (GE) [Domain] |  |
Rate Matters [Abstract] |  |
Percentage of Fault |  | 52.00%
NSP-Minnesota |  |
Rate Matters [Abstract] |  |
Percentage of Fault |  | 48.00%
Customer refund of previously recovered purchased power costs | $ 20 |

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