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Regulatory Matters - Electric Rates (Details) $ in Millions |  |  |  |  |  |  | 3 Months Ended | 4 Months Ended | 8 Months Ended | 12 Months Ended | 18 Months Ended | 24 Months Ended | 36 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 23, 2023 USD ($) | Dec. 20, 2022 USD ($) | Jun. 01, 2022 USD ($) | Jan. 13, 2022 USD ($) | Nov. 01, 2021 USD ($) | Nov. 01, 2019 | Jun. 30, 2021 USD ($) | Apr. 30, 2020 USD ($) | Dec. 31, 2020 | Dec. 31, 2022 USD ($) Years Customers | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2022 USD ($) Years Customers | Aug. 11, 2021 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities - Reserve for Interim Rate Refund |  |  |  |  |  |  |  |  |  |  | $ 18.4 | $ 0.0 |  |  | $ 0.0 | $ 18.4 |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | 441.0 | 511.8 |  |  | 511.8 | 441.0 |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | $ 441.0 | 511.8 |  |  | 511.8 | 441.0 |
Cost Recovery Riders [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alternative Revenue Program, Required Collection Period |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |
Regulatory Assets | [1] |  |  |  |  |  |  |  |  |  | $ 41.2 | 63.2 |  |  | 63.2 | 41.2 |
Regulatory Assets | [1] |  |  |  |  |  |  |  |  |  | 41.2 | 63.2 |  |  | 63.2 | $ 41.2 |
MPUC [Member] | Fuel Adjustment Clause Filing [Member] | Minnesota Power [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forced Outage Costs |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8.0 |  |  |
Administrative Law Judge Recommended Refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.0
Commission Approved Refund |  |  |  |  | $ 5.0 |  |  |  |  |  |  |  |  |  |  |  |
PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.40% |
Approved Equity Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55.00% |
PSCW [Member] | 2022 Wisconsin General Rate Review | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase |  |  |  | 3.60% |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Return on Equity |  |  |  | 10.40% |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Equity Ratio |  |  |  | 55.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  |  | $ 4.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Rate Increase - Amount |  |  | $ 3.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
PSCW [Member] | 2022 Wisconsin General Rate Case | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Equity Ratio |  |  | 55.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from Cost Recovery Riders |  |  |  |  |  |  |  |  |  |  | 38.8 | 38.9 | $ 29.9 |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2020 Minnesota General Rate Review [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase |  |  |  |  |  |  | 10.60% |  |  |  |  |  |  |  |  |  |
Public Utilities, Interim Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  | $ 36.1 |  |  |  |  |  |  |  |
Proposed Ongoing Customer Rate Increase |  |  |  |  |  |  |  |  |  | 4.10% |  |  |  |  |  |  |
Public Utilities, Interim Rate Increase (Decrease), Percentage |  |  |  |  |  |  |  |  | 5.80% |  |  |  |  |  |  |  |
Public Utilities Reserve for Interim Rates (Recorded During Period) |  |  |  |  |  |  |  | $ 11.7 |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2022 Minnesota General Rate Review | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase |  |  |  |  |  | 18.00% |  |  |  |  |  |  |  |  |  |  |
Requested Return on Equity |  |  |  |  |  | 10.25% |  |  |  |  |  |  |  |  |  |  |
Requested Equity Ratio |  |  |  |  |  | 53.81% |  |  |  |  |  |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  |  |  |  | $ 108.0 |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Interim Rate Increase (Decrease), Amount |  |  |  |  |  | 80.0 |  |  |  |  |  |  |  |  |  |  |
Public Utility - Additional Rider Revenue |  |  |  |  |  |  |  |  |  |  | 9.0 |  |  |  |  |  |
Approximate Pre-Tax Reserve for Deferred Residential Customer Interim Rates |  |  |  |  |  |  |  |  |  |  | 8.0 |  |  |  |  |  |
Customer Refund Liability, Current |  |  |  |  |  |  |  |  |  |  | 18.0 |  |  |  |  | $ 18.0 |
Electric Rates [Member] | MPUC [Member] | 2022 Minnesota General Rate Review | Minnesota Power [Member] | Retail Customers [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  | 9.65% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Equity Ratio |  | 52.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Rate Increase - Amount |  | $ 60.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utility - Additional Rider Revenue |  | $ 10.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2022 Minnesota General Rate Review | Minnesota Power [Member] | Residential [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred Interim Rates |  |  |  |  |  | $ 8.0 |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | Cost Recovery Riders [Member] | Minnesota Power [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | 3.8 |  |  |  |  | 3.8 |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | 3.8 |  |  |  |  | 3.8 |
Electric Rates [Member] | MPUC [Member] | Fuel Adjustment Clause | Minnesota Power [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  | 56.0 |  |  | 56.0 |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  | $ 56.0 |  |  | $ 56.0 |  |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Review | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.25% |  |
Approved Equity Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53.81% |  |
Electric Rates [Member] | MPUC [Member] | Fuel Adjustment Clause 2022 | Minnesota Power [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | 13.0 |  |  |  |  | 13.0 |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  | $ 13.0 |  |  |  |  | $ 13.0 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  |  |  |  | 14 |  |  |  |  | 14 |
Notice Required to Terminate (Years) | Years |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  | 3 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Through 2029) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  |  |  |  | 13 |  |  |  |  | 13 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  | 2 |
[1] | The cost recovery rider regulatory assets and liabilities are revenue not yet collected from our customers and cash collections from our customers in excess of the revenue recognized, respectively, primarily due to capital expenditures related to Bison and the GNTL as well as differences between production tax credits recognized and those assumed in Minnesota Power’s base rates. The cost recovery rider regulatory assets as of December 31, 2022, will be recovered within the next two years.

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Name: | ale_LongtermContractsforSaleofElectricPowerAxis=ale_WholesaleElectricContractCostBasedFormulaMethodologyforEntireTermMember
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Data Type: | na
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