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Regulatory Matters - Integrated Resource Plan (Details) - MPUC [Member] - MW | Jul. 28, 2017 | Sep. 30, 2017 | Dec. 31, 2015
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Resource Package [Member] | Tenaska PPA [Member] | Wind Turbine Generators [Member] | Tenaska [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Generating Capacity to be Counterparty Owned (MW) |  | 250 |
Resource Package [Member] | Minimum [Member] | Natural Gas PPA [Member] | Natural Gas-fired [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Combined-Cycle Natural Gas-fired Generating Facility [Member] | Jointly Owned Electricity Generation Plant [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Generating Capacity to be Jointly Owned (MW) | 525 | 525 |
Resource Package [Member] | Maximum [Member] | Natural Gas PPA [Member] | Natural Gas-fired [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Combined-Cycle Natural Gas-fired Generating Facility [Member] | Jointly Owned Electricity Generation Plant [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Generating Capacity to be Jointly Owned (MW) | 550 | 550 |
Minnesota Power [Member] | Integrated Resource Plan [Member] | Minimum [Member] | Natural Gas-fired [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Additional Natural Gas-fired Generation (MW) |  |  | 200
Minnesota Power [Member] | Integrated Resource Plan [Member] | Maximum [Member] | Natural Gas-fired [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Additional Natural Gas-fired Generation (MW) |  |  | 300
Minnesota Power [Member] | Resource Package [Member] | Tenaska PPA [Member] | Wind Turbine Generators [Member] | Tenaska [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Generating Capacity to be Counterparty Owned (MW) | 250 |  |
Minnesota Power [Member] | Resource Package [Member] | Solar Energy PPA [Member] | Solar Energy Generation [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Generating Capacity to be Counterparty Owned (MW) | 10 |  |
Minnesota Power [Member] | Resource Package [Member] | Natural Gas PPA [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Long-Term Contract for Purchase of Electric Power, Share of Plant Output to be Purchased (MW) | 250 |  |
Minnesota Power [Member] | Resource Package [Member] | Natural Gas PPA [Member] | Natural Gas-fired [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Expected Output Entitlement | 50.00% |  |

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