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Equity Investments (Details) $ in Millions |  | 12 Months Ended
--- | --- | ---
Jan. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Beginning Balance |  |  | $ 118.7 |  |
Cash Investments |  |  | 39.2 | $ 7.8 | $ 5.4
Equity Earnings in ATC |  |  | 17.5 | 22.5 | 18.5
Equity Investment Ending Balance |  |  | 161.1 | 118.7 |
Income Statement Data [Abstract] |  |  |  |  |
ALLETE's Equity in Net Income |  |  | $ 17.5 | 22.5 | 18.5
FERC [Member] | Return on Equity Complaint 2 [Member] |  |  |  |  |
Income Statement Data [Abstract] |  |  |  |  |
Proposed Return on Equity |  |  | 9.70% |  |
Proposed Return on Equity, Including Incentive Adder |  |  | 10.20% |  |
ATC [Member] |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Ownership Percentage |  |  | 8.00% |  |
Expected Additional Investment in 2019 |  |  | $ 8.5 |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Beginning Balance |  |  | 118.7 | 135.6 |
Cash Investments |  |  | 6.2 | 7.8 |
Equity Earnings in ATC |  |  | 17.5 | 22.5 | 18.5
Distributed ATC Earnings |  |  | (15.2) | (19.3) |
Remeasurement of Deferred Income Taxes for Equity Method Investments | [1] |  | 0.0 | (27.9) |
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments |  |  | 0.9 | 0.0 |
Equity Investment Ending Balance |  |  | 128.1 | 118.7 | 135.6
Balance Sheet Data [Abstract] |  |  |  |  |
Current Assets |  |  | 87.2 | 87.7 |
Non-Current Assets |  |  | 4,928.8 | 4,598.9 |
Total Assets |  |  | 5,016.0 | 4,686.6 |
Current Liabilities |  |  | 640.0 | 767.2 |
Long-Term Debt |  |  | 2,014.0 | 1,790.6 |
Other Non-Current Liabilities |  |  | 295.3 | 240.3 |
Members' Equity |  |  | 2,066.7 | 1,888.5 |
Total Liabilities and Members' Equity |  |  | 5,016.0 | 4,686.6 |
Income Statement Data [Abstract] |  |  |  |  |
Revenue |  |  | 690.5 | 721.6 | 650.8
Operating Expense |  |  | 358.7 | 344.9 | 322.5
Other Expense |  |  | 108.3 | 104.1 | 95.5
Net Income |  |  | 223.5 | 272.6 | 232.8
ALLETE's Equity in Net Income |  |  | $ 17.5 | $ 22.5 | $ 18.5
Authorized Return on Equity |  |  | 10.32% |  |
Authorized Return on Equity, Including Incentive Adder |  |  | 10.82% |  |
ATC [Member] | Subsequent Event [Member] |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Cash Investments |  | $ 0.4 |  |  |
Nobles 2 [Member] |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Ownership Percentage |  |  | 49.00% |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Ending Balance |  |  | $ 33.0 |  |
Nobles 2 [Member] | Minimum [Member] |  |  |  |  |
Income Statement Data [Abstract] |  |  |  |  |
Estimated Project Cost |  |  | 350.0 |  |
Portion of Expected Project Costs |  |  | 170.0 |  |
Expected Tax Equity Financing |  |  | 60.0 |  |
Nobles 2 [Member] | Maximum [Member] |  |  |  |  |
Income Statement Data [Abstract] |  |  |  |  |
Estimated Project Cost |  |  | 400.0 |  |
Portion of Expected Project Costs |  |  | 200.0 |  |
Expected Tax Equity Financing |  |  | $ 70.0 |  |
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member] | MPUC [Member] | Resource Package [Member] |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Generating Capacity to be Counterparty Owned (MW) | MW | [2] |  | 250 |  |
Contract Term (Years) |  |  | 20 years |  |
[1] | Impact of the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.
[2] | The agreement includes a fixed capacity charge and energy prices that escalate at a fixed rate annually over the term.

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