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Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
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Cash, Cash Equivalents and Restricted Cash [Abstract] |  |  |  |  |
Cash and Cash Equivalents |  | $ 159.0 | $ 44.3 | $ 67.0 | $ 69.3
Restricted Cash included in Prepayments and Other |  | 7.1 | 0.8 | 6.1 | 2.8
Restricted Cash, Non-Current |  | 10.9 | 20.1 | 5.8 | 20.4
Total Cash, Cash Equivalents and Restricted Cash |  | 177.0 | 65.2 | 78.9 | 92.5
Inventories – Net [Abstract] |  |  |  |  |
Fuel | [1] | 20.4 | 23.1 |  |
Materials and Supplies |  | 53.6 | 51.1 |  |
Total Inventories – Net |  | 74.0 | 74.2 |  |
Other Non-Current Assets [Abstract] |  |  |  |  |
Contract Assets | [2] | 24.8 | 25.5 |  |
Operating Lease, Right-of-Use Asset |  | 20.2 | 22.4 |  |
ALLETE Properties |  | 18.2 | 18.2 |  |
Restricted Cash, Non-Current |  | 10.9 | 20.1 | $ 5.8 | $ 20.4
Other Postretirement Benefit Plans |  | 34.7 | 34.2 |  |
Other |  | 82.5 | 86.4 |  |
Total Other Non-Current Assets |  | 191.3 | 206.8 |  |
Other Current Liabilites [Abstract] |  |  |  |  |
PSAs |  | 12.5 | 12.5 |  |
Public Utilities, Fuel Adjustment Clause |  | 1.8 | 3.7 |  |
Operating Lease Liabilities |  | 5.7 | 5.9 |  |
Other |  | 58.9 | 44.6 |  |
Total Other Current Liabilities |  | 78.9 | 66.7 |  |
Other Non-Current Liabilities [Abstract] |  |  |  |  |
Asset Retirement Obligation (a) | [3] | 170.8 | 166.6 |  |
PSAs |  | 48.9 | 52.1 |  |
Operating Lease Liabilities |  | 14.5 | 16.5 |  |
Other |  | 46.0 | 50.1 |  |
Total Other Non-Current Liabilities |  | 280.2 | 285.3 |  |
BNI Energy |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |
Other Receivables |  | $ 25.0 | $ 25.0 |  |
[1] | Fuel consists primarily of coal inventory at Minnesota Power.
[2] | Contract Assets consist of payments made to customers as an incentive to execute or extend service agreements. The contract payments are being amortized over the term of the respective agreements as a reduction to revenue.
[3] | The asset retirement obligation is primarily related to our Regulated Operations and is funded through customer rates over the life of the related assets. Additionally, BNI Energy funds its obligation through its cost-plus coal supply agreements for which BNI Energy has recorded a receivable of $25.0 million in Other Non-Current Assets on the Consolidated Balance Sheet as of March 31, 2021 and December 31, 2020.

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