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REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory assets
Current assets	$ 30.3	$ 37.2
Regulatory assets	3,643.5	2,803.2
Total regulatory assets	3,673.8	2,840.4
Unrecognized pension and OPEB costs
Regulatory assets
Total regulatory assets	1,071.0	1,142.0
Plant retirements
Regulatory assets
Total regulatory assets	778.4	15.1
Environmental remediation costs
Regulatory assets
Total regulatory assets	680.0	676.6
System support resource
Regulatory assets
Total regulatory assets	315.0	298.9
Income tax
Regulatory assets
Total regulatory assets	272.0	15.7
Asset retirement obligations
Regulatory assets
Total regulatory assets	224.1	192.2
Electric transmission costs
Regulatory assets
Total regulatory assets	124.0	221.0
We Power generation
Regulatory assets
Total regulatory assets	48.7	71.3
Uncollectible expense
Regulatory assets
Total regulatory assets	39.3	35.1
Energy efficiency programs
Regulatory assets
Total regulatory assets	15.7	24.6
Other, net
Regulatory assets
Total regulatory assets	$ 105.6	$ 147.9

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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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.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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