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OPERATING REVENUES - OTHER NON-UTILITY OPERATING REVENUES (Details) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2018 MW	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
We Power revenues
Disaggregation of Operating Revenues
Revenues amortized from deferred revenue during the period		$ 6.4	$ 19.0
Appliance service repairs \| Maximum
Disaggregation of Operating Revenues
Duration of contract for remaining performance obligations in contract		1 year	1 year
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers		$ 1,635.7	$ 5,599.3
Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers		13.6	38.2
Other non-utility revenues \| Revenues from contracts with customers \| We Power revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers		6.4	19.0
Other non-utility revenues \| Revenues from contracts with customers \| Distributed renewable solar project revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers		2.3	6.4
Other non-utility revenues \| Revenues from contracts with customers \| Wind generation revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers		0.8	0.8
Other non-utility revenues \| Revenues from contracts with customers \| Other
Disaggregation of Operating Revenues
Revenues from contracts with customers		0.1	0.2
Transferred over time \| Other non-utility revenues \| Revenues from contracts with customers \| Appliance service repairs
Disaggregation of Operating Revenues
Revenues from contracts with customers		$ 4.0	$ 11.8
Bishop Hill III Wind Energy Center
Disaggregation of Operating Revenues
WEC's ownership interest in Bishop Hill III Wind Energy Center	80.00%
Capacity of Bishop Hill III Wind Energy Center \| MW	132
Duration of offtake agreement for the sale of energy produced	22 years