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Regulatory Matters - Conservation Improvement Program (CIP) (Details) - MPUC [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
CIP Triennial Filing [Member] | Minnesota Power [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
CIP Investment Goal | $ 10.3 | $ 10.3 | $ 7.3
CIP Actual Spending | 9.0 | 8.1 | 7.4
CIP Investment Goal in 2019 | 10.5 |  |
CIP Consolidated Filing [Member] | Minnesota Power [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
CIP Financial Incentive | $ 3.0 | $ 5.5 | $ 7.5
Minimum [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
CIP Spending Minimum - Percentage | 1.50% |  |

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- Definition Conservation improvement program actual annual spend. + References No definition available. + Details
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- Definition Minnesota Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. + References No definition available. + Details
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