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Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions | Jun. 30, 2016 | Dec. 31, 2015
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Inventories [Abstract] |  |  |
Fuel | [1] | $ 49.2 | $ 58.1
Materials and Supplies |  | 49.8 | 49.1
Raw Materials |  | 2.8 | 2.7
Work in Progress |  | 0.6 | 0.0
Finished Goods |  | 8.3 | 7.5
Reserve for Obsolescence |  | (0.3) | (0.3)
Total Inventories |  | 110.4 | 117.1
Prepayments and Other Current Assets [Abstract] |  |  |
Deferred Fuel Adjustment Clause |  | 14.5 | 10.6
Restricted Cash | [2] | 7.5 | 5.6
Other |  | 16.4 | 19.5
Total Prepayments and Other Current Assets |  | 38.4 | 35.7
Other Non-Current Assets [Abstract] |  |  |
Restricted Cash |  | 8.2 | 8.1
Contract Payment |  | 31.0 |
Other Current Liabilites [Abstract] |  |  |
Customer Deposits |  | 13.4 | 15.1
Power Purchase Agreements |  | 23.9 | 23.3
Other |  | 48.6 | 47.7
Total Other Current Liabilities |  | 85.9 | 86.1
Other Non-Current Liabilities [Abstract] |  |  |
Asset Retirement Obligation |  | 135.2 | 131.4
Power Purchase Agreements |  | 125.9 | 138.1
Contingent Consideration | [3] | 37.3 | 36.6
Other |  | 42.4 | 42.9
Total Other Non-Current Liabilities |  | $ 340.8 | $ 349.0
[1] | Fuel consists primarily of coal inventory at Minnesota Power.
[2] | Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and cash pledged as collateral for U.S. Water Services’ standby letters of credit.
[3] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 3. Acquisitions and Note 5. Fair Value.)

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