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Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019
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Cash, Cash Equivalents and Restricted Cash [Abstract] |  |  |  |  |
Cash and Cash Equivalents |  | $ 59.0 | $ 44.3 | $ 79.0 | $ 69.3
Restricted Cash included in Prepayments and Other |  | 4.1 | 0.8 | 3.2 | 2.8
Restricted Cash included in Other Non-Current Assets |  | 2.3 | 20.1 | 2.9 | 20.4
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows |  | 65.4 | 65.2 | 85.1 | 92.5
Inventories – Net [Abstract] |  |  |  |  |
Fuel | [1] | 12.4 | 23.1 |  |
Materials and Supplies |  | 55.2 | 51.1 |  |
Construction of Wind Energy Facilities | [2] | 22.0 | 0.0 |  |
Total Inventories – Net |  | 89.6 | 74.2 |  |
Other Non-Current Assets [Abstract] |  |  |  |  |
Contract Assets | [3] | 23.4 | 25.5 |  |
Operating Lease, Right-of-Use Asset |  | 17.7 | 22.4 |  |
ALLETE Properties |  | 17.6 | 18.2 |  |
Restricted Cash, Non-Current |  | 2.3 | 20.1 | $ 2.9 | $ 20.4
Other Postretirement Benefit Plans |  | 35.8 | 34.2 |  |
Other |  | 82.9 | 86.4 |  |
Total Other Non-Current Assets |  | 179.7 | 206.8 |  |
Other Current Liabilites [Abstract] |  |  |  |  |
Customer Deposits | [4] | 27.5 | 7.4 |  |
PSAs |  | 12.6 | 12.5 |  |
Fuel Adjustment Clause |  | 2.6 | 3.7 |  |
Operating Lease Liabilities |  | 5.0 | 5.9 |  |
Other |  | 36.3 | 37.2 |  |
Total Other Current Liabilities |  | 84.0 | 66.7 |  |
Other Non-Current Liabilities [Abstract] |  |  |  |  |
Asset Retirement Obligation | [5] | 170.9 | 166.6 |  |
PSAs |  | 42.6 | 52.1 |  |
Operating Lease Liabilities |  | 12.7 | 16.5 |  |
Other |  | 51.9 | 50.1 |  |
Total Other Non-Current Liabilities |  | 278.1 | 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] | Project costs related to ALLETE Clean Energy’s Northern Wind and Red Barn wind projects which are expected be sold in late 2022. (See Other Current Liabilities.)
[3] | 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.
[4] | Primarily related to deposits received by ALLETE Clean Energy for the Northern Wind and Red Barn wind projects which are expected be sold in late 2022. (See Inventories – Net.)
[5] | 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 September 30, 2021, and December 31, 2020.

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