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Regulatory Matters - Nemadji Trail Energy Center (Details) $ in Millions | Jul. 28, 2017 MW | Jun. 30, 2020 USD ($)
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Nemadji Trail Energy Center [Member] | Maximum [Member] |  |
Regulatory Matters [Line Items] |  |
Estimated Capital Expenditures, Including Past Expenditures | $ |  | $ 700
ALLETE, Inc. [Member] | Nemadji Trail Energy Center [Member] |  |
Regulatory Matters [Line Items] |  |
Estimated Capital Expenditures, Including Past Expenditures | $ |  | 350
ALLETE, Inc. [Member] | Nemadji Trail Energy Center [Member] | Maximum [Member] |  |
Regulatory Matters [Line Items] |  |
Total Project Costs Incurred to Date | $ |  | $ 14
MPUC [Member] | Resource Package [Member] | Natural Gas-fired [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Minimum [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member] | Jointly Owned Electricity Generation Plant [Member] |  |
Regulatory Matters [Line Items] |  |
Generating Capacity to be Jointly Owned (MW) | MW | 525 |
MPUC [Member] | Resource Package [Member] | Natural Gas-fired [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Maximum [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member] | Jointly Owned Electricity Generation Plant [Member] |  |
Regulatory Matters [Line Items] |  |
Generating Capacity to be Jointly Owned (MW) | MW | 550 |
MPUC [Member] | Resource Package [Member] | Minnesota Power [Member] | Natural Gas PPA [Member] |  |
Regulatory Matters [Line Items] |  |
Output Being Purchased (MW) | MW | 250 |
MPUC [Member] | Resource Package [Member] | Minnesota Power [Member] | Natural Gas-fired [Member] | Natural Gas PPA [Member] |  |
Regulatory Matters [Line Items] |  |
Expected Output Entitlement (Percent) | 50.00% |

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