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Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accounts Receivable [Abstract] |  |  |  |
Billed |  | $ 121.7 | $ 112.6 |
Unbilled |  | 24.4 | 24.6 |
Less: Allowance for Doubtful Accounts |  | 1.7 | 2.1 |
Total Accounts Receivable |  | 144.4 | 135.1 |
Inventories – Net [Abstract] |  |  |  |
Fuel | [1] | 26.0 | 34.8 |
Materials and Supplies |  | 44.2 | 46.5 |
Raw Materials |  | 2.8 | 2.8 |
Work in Progress |  | 6.1 | 4.2 |
Finished Goods |  | 8.4 | 8.3 |
Reserve for Obsolescence |  | (0.8) | (0.7) |
Total Inventories |  | 86.7 | 95.9 |
Impairment of Long-Lived Assets [Abstract] |  |  |  |
Impairment of Real Estate |  | 0.0 | 0.0 | $ 0.0
Goodwill [Line Items] |  |  |  |
Impairment of Goodwill | [2] | 0.0 | 0.0 | 3.3
Definite-Lived Intangible Assets [Line Items] |  |  |  |
Provision for Interim Rate Refunds |  | 40.0 | 0.0 |
Provision for Tax Reform Refunds |  | 10.7 | 0.0 |
Indefinite-Lived Intangible Assets [Abstract] |  |  |  |
Impairment of Intangible Assets |  | 0.0 | 0.0 | 0.0
Other Non-Current Assets [Abstract] |  |  |  |
Contract Asset - Noncurrent | [3] | 30.7 | 31.6 |
Financing Receivable | [4] | 10.4 | 11.0 |
Other |  | 62.2 | 65.1 |
Total Other Non-Current Assets |  | 103.3 | 107.7 |
Other Current Liabilities [Abstract] |  |  |  |
PSAs |  | 12.6 | 24.5 |
Contract with Customer, Liability, Current | [5] | 7.6 | 8.7 |
Business Combination, Contingent Consideration, Liability, Current | [6] | 3.8 | 0.0 |
Other |  | 53.8 | 50.0 |
Total Other Current Liabilities |  | 128.5 | 83.2 |
Other Non-Current Liabilities [Abstract] |  |  |  |
Asset Retirement Obligation |  | 138.6 | 122.7 |
PSAs |  | 76.9 | 89.5 |
Contingent Consideration | [7] | 0.0 | 5.4 |
Other |  | 47.1 | 49.5 |
Total Other Non-Current Liabilities |  | 262.6 | 267.1 |
Business Segments [Line Items] |  |  |  |
Other – Non-utility |  | 23.6 | 23.6 | 22.3
Operating Expenses – Other [Abstract] |  |  |  |
Impairment of Real Estate |  | 0.0 | 0.0 | 0.0
Impairment of Goodwill | [2] | 0.0 | 0.0 | 3.3
Change in Fair Value of Contingent Consideration | [8] | (2.0) | (0.7) | (13.6)
Total Operating Expenses – Other |  | $ (2.0) | $ (0.7) | $ (10.3)
Income Taxes [Abstract] |  |  |  |
More-Likely-Than-Not Percentage |  | 50.00% |  | 50.00%
Remeasurement of Deferred Income Taxes as a Result of the TCJA [Line Items] |  |  |  |
Statutory Federal Income Tax Rate |  | 21.00% | 35.00% | 35.00%
Deferred Income Tax Expense (Benefit) |  | $ (15.8) | $ 14.4 | $ 19.4
Decrease in Investment in ATC |  | 0.0 | (27.9) | 0.0
Increase to Regulatory Assets |  | 0.0 | 80.9 | 0.0
Increase to Regulatory Liabilities |  | 0.0 | 393.6 | 0.0
ALLETE Clean Energy [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Impairment of Goodwill |  |  |  | 3.3
Operating Expenses – Other [Abstract] |  |  |  |
Impairment of Goodwill |  |  |  | 3.3
Total Operating Expenses – Other | [9] | 0.0 | 0.0 | 3.3
U.S. Water Services [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Impairment of Goodwill |  | 0.0 | 0.0 | 0.0
Operating Expenses – Other [Abstract] |  |  |  |
Impairment of Goodwill |  | 0.0 | 0.0 | 0.0
Corporate and Other [Member] |  |  |  |
Operating Expenses – Other [Abstract] |  |  |  |
Total Operating Expenses – Other | [9] | $ (2.0) | $ (0.7) | $ (13.6)
Discount Rate [Member] | U.S. Water Services [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Discounted Cash Flow Model Significant Assumptions |  | 0.120 |  |
Annual Revenue Growth Rate [Member] | U.S. Water Services [Member] | Minimum [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Discounted Cash Flow Model Significant Assumptions |  | .060 |  |
Annual Revenue Growth Rate [Member] | U.S. Water Services [Member] | Maximum [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Discounted Cash Flow Model Significant Assumptions |  | .100 |  |
Measurement Input, Terminal Growth Rate [Member] | U.S. Water Services [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Discounted Cash Flow Model Significant Assumptions |  | 0.035 |  |
Developed Technology and Other [Member] | Minimum [Member] |  |  |  |
Definite-Lived Intangible Assets [Line Items] |  |  |  |
Useful Life (Years) |  | 4 years |  |
Developed Technology and Other [Member] | Maximum [Member] |  |  |  |
Definite-Lived Intangible Assets [Line Items] |  |  |  |
Useful Life (Years) |  | 10 years |  |
Customer Relationships [Member] |  |  |  |
Definite-Lived Intangible Assets [Line Items] |  |  |  |
Useful Life (Years) |  | 19 years |  |
[1] | Fuel consists primarily of coal inventory at Minnesota Power.
[2] | See Goodwill and Intangible Assets.
[3] | Contract Assets include 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] | Finance Receivable reflects the remaining balance due from the ALLETE Properties sale of its Ormond Crossings project and Lake Swamp wetland mitigation bank.
[5] | Contract Liabilities include deposits received as a result of entering into contracts with our customers prior to completing our performance obligations.
[6] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 9. Fair Value.)
[7] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 9. Fair Value.)
[8] | See Note 9. Fair Value.
[9] | See Note 1. Operations and Significant Accounting Policies.

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