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Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
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Inventories – Net [Abstract] |  |  |
Fuel | [1] | $ 45.0 | $ 43.9
Materials and Supplies |  | 48.0 | 48.7
Raw Materials |  | 2.8 | 2.9
Work in Progress |  | 0.3 | 1.0
Finished Goods |  | 8.3 | 8.6
Reserve for Obsolescence |  | (1.0) | (0.9)
Total Inventories – Net |  | 103.4 | 104.2
Prepayments and Other Current Assets [Abstract] |  |  |
Deferred Fuel Adjustment Clause |  | 16.7 | 18.6
Restricted Cash |  | 3.7 | 2.2
Other |  | 18.4 | 19.5
Total Prepayments and Other Current Assets |  | 38.8 | 40.3
Other Non-Current Assets [Abstract] |  |  |
Contract Payment |  | 28.4 | 29.6
Finance Receivable |  | 11.5 | 11.5
Restricted Cash |  | 8.6 | 8.6
Other |  | 56.3 | 56.8
Total Other Non-Current Assets |  | 104.8 | 106.5
Other Current Liabilites [Abstract] |  |  |
PSAs |  | 24.7 | 24.6
Other |  | 45.4 | 49.1
Total Other Current Liabilities |  | 70.1 | 73.7
Other Non-Current Liabilities [Abstract] |  |  |
Asset Retirement Obligation |  | 158.2 | 136.6
PSAs |  | 101.5 | 113.8
Contingent Consideration | [2] | 5.5 | 25.0
Other |  | 43.9 | 47.3
Total Other Non-Current Liabilities |  | $ 309.1 | $ 322.7
[1] | Fuel consists primarily of coal inventory at Minnesota Power.
[2] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 5. Fair Value.)

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