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Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Regulatory liabilities |  |
Current liabilities | $ 36.8 | $ 41.8
Regulatory liabilities | 4,251.6 | 3,718.6
Total regulatory liabilities | 4,288.4 | 3,760.4
Income tax related items |  |
Regulatory liabilities |  |
Total regulatory liabilities | 2,406.6 | 2,134.1
Removal costs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 1,329.6 | 1,294.9
Pension and OPEB costs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 238.3 | 114.2
Mines deferral |  |
Regulatory liabilities |  |
Total regulatory liabilities | 120.8 | 95.1
Energy costs refundable through rate adjustments |  |
Regulatory liabilities |  |
Total regulatory liabilities | 39.6 | 42.0
Energy efficiency programs |  |
Regulatory liabilities |  |
Total regulatory liabilities | 31.7 | 21.1
Uncollectible expense |  |
Regulatory liabilities |  |
Total regulatory liabilities | 30.5 | 24.7
Decoupling |  |
Regulatory liabilities |  |
Total regulatory liabilities | 30.5 | 1.8
Earnings sharing mechanisms |  |
Regulatory liabilities |  |
Total regulatory liabilities | 30.0 | 2.5
Derivatives |  |
Regulatory liabilities |  |
Total regulatory liabilities | 16.4 | 11.0
Other, net |  |
Regulatory liabilities |  |
Total regulatory liabilities | $ 14.4 | $ 19.0

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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