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REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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Regulatory assets |  |
Current assets | $ 27.3 | $ 50.7
Regulatory assets | 4,009.8 | 3,805.1
Total regulatory assets | 4,037.1 | 3,855.8
Pension and OPEB costs |  |
Regulatory assets |  |
Total regulatory assets | 1,173.8 | 1,193.5
Plant retirements |  |
Regulatory assets |  |
Total regulatory assets | 1,027.9 | 832.3
Environmental remediation costs |  |
Regulatory assets |  |
Total regulatory assets | 697.6 | 687.1
Income tax related items |  |
Regulatory assets |  |
Total regulatory assets | 384.8 | 369.1
System support resource |  |
Regulatory assets |  |
Total regulatory assets | 317.9 | 316.7
Asset retirement obligations |  |
Regulatory assets |  |
Total regulatory assets | 223.4 | 185.4
Uncollectible expense |  |
Regulatory assets |  |
Total regulatory assets | 47.6 | 38.7
Electric transmission costs |  |
Regulatory assets |  |
Total regulatory assets | 41.7 | 58.1
We Power generation |  |
Regulatory assets |  |
Total regulatory assets | 36.1 | 43.0
Energy efficiency programs |  |
Regulatory assets |  |
Total regulatory assets | 11.3 | 14.0
Other, net |  |
Regulatory assets |  |
Total regulatory assets | $ 75.0 | $ 117.9

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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