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REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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Regulatory liabilities |  |
Current liabilities | $ 103.9 | $ 36.8
Regulatory liabilities | 4,274.3 | 4,251.6
Total regulatory liabilities | 4,378.2 | 4,288.4
Income tax related items |  |
Regulatory liabilities |  |
Total regulatory liabilities | 2,401.2 | 2,406.6
Removal costs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 1,330.8 | 1,329.6
Pension and OPEB costs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 235.7 | 238.3
Mines deferral |  |
Regulatory liabilities |  |
Total regulatory liabilities | 129.1 | 120.8
Energy costs refundable through rate adjustments |  |
Regulatory liabilities |  |
Total regulatory liabilities | 103.0 | 39.6
Decoupling |  |
Regulatory liabilities |  |
Total regulatory liabilities | 51.0 | 30.5
Energy efficiency programs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 45.7 | 31.7
Earnings sharing mechanisms |  |
Regulatory liabilities |  |
Total regulatory liabilities | 29.9 | 30.0
Uncollectible expense |  |
Regulatory liabilities |  |
Total regulatory liabilities | 29.5 | 30.5
Derivatives |  |
Regulatory liabilities |  |
Total regulatory liabilities | 9.0 | 16.4
Other, net |  |
Regulatory liabilities |  |
Total regulatory liabilities | $ 13.3 | $ 14.4

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