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Regulatory Assets and Liabilities - Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Regulatory Liabilities |  |
Current liabilities | $ 41.8 | $ 33.4
Regulatory liabilities | 3,718.6 | 1,563.8
Total regulatory liabilities | 3,760.4 | 1,597.2
2017 Tax legislation impact and income tax related |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 2,134.1 | 0.0
Removal costs |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 1,294.9 | 1,262.7
Unrecognized pension and OPEB costs |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 114.2 | 63.0
Mines deferral |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 95.1 | 70.2
Energy costs refundable through rate adjustments |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 42.0 | 88.7
Uncollectible expense |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 24.7 | 36.1
Derivatives |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 11.0 | 41.1
Other, net |  |
Regulatory Liabilities |  |
Total regulatory liabilities | 44.4 | $ 35.4
Utility operations |  |
Regulatory Liabilities |  |
Increase (decrease) in deferred income taxes | $ 2,450.0 |

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