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Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Trade Accounts Receivable [Abstract] |  |  |  |  |
Billed |  | $ 105.3 |  | $ 85.5 |
Unbilled |  | 16.9 |  | 18.6 |
Less: Allowance for Doubtful Accounts |  | 1.0 |  | 1.1 |
Total Accounts Receivable |  | 121.2 |  | 103.0 |
Inventories [Abstract] |  |  |  |  |
Fuel | [1] | 58.1 |  | 29.0 |
Materials and Supplies |  | 49.1 |  | 51.5 |
Raw Materials |  | 2.7 |  | 0.0 |
Finished Goods |  | 7.5 |  | 0.0 |
Reserve for Obsolescence |  | (0.3) |  | 0.0 |
Total Inventories |  | 117.1 |  | 80.5 |
Prepayments and Other Current Assets [Abstract] |  |  |  |  |
Deferred Fuel Adjustment Clause |  | 10.6 |  | 16.3 |
Construction Costs for Development Project | [2] | 0.0 |  | 48.2 |
Restricted Cash | [3] | 5.6 |  | 2.7 |
Other Assets |  | 19.5 |  | 14.8 |
Total Prepayments and Other Current Assets |  | 35.7 |  | 82.0 |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Remaining Net Book Value |  | 3,669.1 |  | 3,284.8 |
Impairment of Long-Lived Assets [Abstract] |  |  |  |  |
Total Carrying Value of Real Estate Assets Before Impairment Charge |  | 83.3 |  |  |
Impairment of Real Estate |  | 36.3 |  | 0.0 | $ 0.0
Other Non-Current Assets [Abstract] |  |  |  |  |
Restricted Cash |  | 8.1 |  | 5.3 |
Other Current Liabilities [Abstract] |  |  |  |  |
Customer Deposits |  | 15.1 |  | 19.7 |
Power Purchase Agreements | [4] | 23.3 |  | 19.4 |
Construction Deposits Received for Development Project | [5] | 0.0 |  | 54.3 |
Other |  | 47.7 | [6] | 27.4 |
Total Other Current Liabilities |  | 86.1 |  | 120.8 |
Customer Refund Liabilities |  | 7.0 |  |  |
Other Non-Current Liabilities [Abstract] |  |  |  |  |
Asset Retirement Obligation |  | 131.4 |  | 109.2 |
Power Purchase Agreements | [7] | 138.1 |  | 110.7 |
Contingent Consideration | [8] | 36.6 |  | 0.0 |
Other |  | 42.9 |  | 45.1 |
Total Other Non-Current Liabilities |  | 349.0 |  | 265.0 |
Business Segments [Line Items] |  |  |  |  |
Amortization of Power Purchase Agreements |  | $ 23.2 |  | 12.7 | 0.0
Income Taxes [Abstract] |  |  |  |  |
More-Likely-Than-Not Percentage |  | 50.00% |  |  |
Current Deferred Income Tax Assets Reclassified to Non-Current Deferred Income Tax Liabilities |  | $ 39.4 |  |  |
Regulated Operations [Member] |  |  |  |  |
Prepayments and Other Current Assets [Abstract] |  |  |  |  |
Construction Costs for Development Project |  | 101.2 |  | 355.4 |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Remaining Net Book Value |  | $ 3,114.1 |  | 2,998.5 |
Business Segments [Line Items] |  |  |  |  |
Alternative Revenue Program, Required Collection Period |  | 24 months |  |  |
ALLETE Clean Energy [Member] |  |  |  |  |
Prepayments and Other Current Assets [Abstract] |  |  |  |  |
Construction Costs for Development Project |  | $ 4.0 |  | 1.3 |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Remaining Net Book Value |  | 447.3 |  | 196.6 |
Business Segments [Line Items] |  |  |  |  |
Amortization of Power Purchase Agreements |  | 23.2 |  | 12.7 |
Corporate and Other [Member] |  |  |  |  |
Prepayments and Other Current Assets [Abstract] |  |  |  |  |
Construction Costs for Development Project |  | 4.5 |  | 4.6 |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Remaining Net Book Value |  | 95.5 |  | 89.7 |
Impairment of Long-Lived Assets [Abstract] |  |  |  |  |
Impairment of Real Estate |  | $ 36.3 |  | $ 0.0 | $ 0.0
Business Segments [Line Items] |  |  |  |  |
Minimum Cash Collections Requirement for Real Estate Sales (Percent) |  | 20.00% |  |  |
Minimum [Member] |  |  |  |  |
Definite-Lived Intangible Assets [Line Items] |  |  |  |  |
Useful Life (Years) |  | 3 years |  |  |
Maximum [Member] |  |  |  |  |
Definite-Lived Intangible Assets [Line Items] |  |  |  |  |
Useful Life (Years) |  | 22 years |  |  |
Taconite Harbor [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Remaining Net Book Value |  | $ 100.0 |  |  |
[1] | Fuel consists primarily of coal inventory at Minnesota Power. During 2015, Minnesota Power increased its coal inventory to take advantage of favorable pricing.
[2] | Construction Costs for Development Project related to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)
[3] | Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and collateral deposits required for U.S. Water Services’ standby letters of credit.
[4] | Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)
[5] | Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)
[6] | Increase in 2015 includes customer refund liabilities of approximately $7 million, pending regulatory approval.
[7] | Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)
[8] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 7. Acquisitions and Note 10. Fair Value.)

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