OPERATING REVENUES (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Operating Revenues
Operating revenues	$ 320.9	$ 350.1	$ 716.7	$ 743.7
Utility segment
Disaggregation of Operating Revenues
Operating revenues	320.9	350.1	716.7	743.7
Utility segment \| Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(0.1)	0.7	1.1	1.6
Utility segment \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	321.0	349.4	715.6	742.1
Utility segment \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	264.6	292.5	541.6	574.6
Utility segment \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 56.4	$ 56.9	$ 174.0	$ 167.5

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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